- “Don't you ever, ever call Rainbow Monkeys stupid!!”
- ―Numbuh 3
Kuki "Beetles" Sanban (三番茎) a.k.a. Numbuh 3 is a pretty happy-go-lucky, 10-year-old 4th-grade Japanese-American girl who is in charge of Diversionary Tactics and is the Medical Specialist of Sector V. She also takes care of the hamsters that power the treehouse. She is the youngest operative, and one of the two female members (alongside Numbuh 5) and the only Asian-American in her sector. As an adult, she becomes President of the Rainbow Monkey Corporation, and the first thing she did when she took over was tear down a Rainbow Monkey Potty training amusement park.
Kuki is a skinny girl of Japanese descent. Her usual outfit is an over-sized green sweater, thin black tights, green socks, and black and white sneakers. She has long straight black hair with bangs. Her eyes are very thin, almost appearing closed most of the time, although she does open them wider on occasion. She has more alternative costumes than any other character in the show.
Numbuh 3 is described as the "Diversionary Tactics Expert," and is the nurse of the medical lab and the primary caretaker of hamsters in Sector V. She has a very short attention span, but is long on charm, speaking in a slight Asian accent, and often alludes to her Japanese heritage. She is girly, optimistic, bubbly, dizzy-headed, clueless, kind, caring, patient, gentle, and arguably smarter than her teammate, Numbuh 4, with whom she harbors a mutual crush seen throughout the show, though it is less obvious.
She loves Rainbow Monkeys, which she collects, fantasizes about, and has extensive knowledge about. Due to her great love for Rainbow Monkeys, she will often irrationally defend them, occasionally allowing her insanely aggressive side to show (generally accompanied by her face turning red, teeth and ears becoming pointed and eyes flaming, resembling an oni, demons from Japanese mythology). This was first seen in Operation: L.I.C.E., when the insects in question had all but destroyed her stuffed animal collection. Despite being generally good natured, she will occasionally have similar explosive tantrums when she is ignored or doesn't get what she wants. As shown when the macaroni Rainbow Monkey painting is accidentally shattered by Numbuh 13 in Operation: M.A.C.A.R.R.O.N.I., she has the ability to scream at a deafening volume.
She is usually seen with a Rainbow Monkey doll, and her room in Sector V's Treehouse is filled with various stuffed animals and anything else colorful and fuzzy. A special ability of Numbuh 3's is that she can smell and sense Rainbow Monkeys with her nose and her heart as stated in Operation: R.A.I.N.B.O.W.S. and Operation: S.A.T.U.R.N.. She seems to also have a capability of speaking languages that would normally be dubbed gibberish or non-understandable, shown in Operation: D.I.A.P.E.R. and Operation: V.I.D.E.O.G.A.M.E. (Baby and Firefly, respectively).
She is the pilot of Hippy-Hop, a giant mechanical rabbit that is equipped with a complete arsenal of weapons such as missiles and drills, with a running gag throughout the show that Hippy-Hop gets destroyed whenever it appears, followed by Numbuh 3 ejecting from the wreck with a parachute, despite being close to the ground. In spite of her attitude, Numbuh 3 shows her intelligence in a few episodes, such in Operation: H.O.L.I.D.A.Y., when it is apparent that she can pilot an airplane. In Operation: H.O.M.E., it was revealed that she had a Rainbow Monkey called My First Rainbow Monkey. Also, in Operation: L.I.C.E., she had a stuffed animal named Mr. Mopsey.
Like Numbuh 5, Numbuh 3 also has a motherly side, as shown in Operation: C.A.M.P.. In this episode, she and Numbuh 2 take care of a skunk named Bradley, referring to the skunk as her baby. She often worries over Bradley and does whatever it takes to make sure that he is safe. Another example of her motherly side was shown in Operation: C.A.K.E.D.-T.H.R.E.E., when she helped Numbuh 1 take care of the baby chicks that had hatched.
- Green Shirt (Main Outfit)
- Nurse outfit
- Pink party dress (From Operation: D.A.T.E.)
- Space outfit (From Operation: S.A.T.U.R.N. and Operation: P.L.A.N.E.T.)
- Rainbow Monkey costume
- Halloween costume (from Operation: T.R.I.C.K.Y.)
- Purple Pyjamas
- White Party dress (From Operation: I.-S.C.R.E.A.M.)
- Red and White outfit (From Operation: C.A.B.L.E.T.V.)
- Pink School play outfit (From Operation: L.O.V.E.)
- Three Swimsuits (Green, Purple, and Pink)
- Investigator outfit (from Operation: C.R.I.M.E.)
- Wedding Dress (From Operation: B.E.A.C.H. and Operation: K.A.S.T.L.E.).
- Blue Airliner uniform (from Operation: H.O.L.I.D.A.Y.)
- Pink winter jacket, bobble hat and dark pink boots (from Operation: S.N.O.W.I.N.G.)
- Green winter jacket, bobble hat and black boots (from Operation: A.R.C.T.I.C.)
- Rainbow Monkey jacket (from Operation: C.L.O.S.E.T.)
- Green chicken finger outfit (from Operation: N.U.G.G.E.T.)
- Super heroine outfit (from Operation: L.E.A.D.E.R.)
- Yellow rainbow Monkey T-shirt (from Operation: R.A.I.N.B.O.W.S.)
- Nigel Uno disguise (from Operation: R.E.C.E.S.S.)
Her most prized characteristic, according to Operation: C.A.R.A.M.E.L., is her cheerfulness, which is "almost too sweet."
- Her favourite type of Rainbow Monkey is an orange one, referencing her crush on Wallabee, specifically his orange hoodie.
- Her favourite snack is popcorn, as shown in No P in the Ool, Operation: S.L.U.M.B.E.R. and Operation: P.L.A.N.E.T.
- Sometimes, when she's really angry, she pulls Numbuh 4 by the shirt.
- Her codename is linked to her last name "Sanban", which is Japanese for "Number 3"; san (三) means three and ban (番) means number.
- Her stuffed animals have the number 3 painted on them.
- She is the 23rd person to steal Grim's scythe, even though she technically received it through legal means.
- The only time she got to lead a mission was Operation: F.A.S.T.-F.O.O.D..
- She was temporarily Sector V's leader in Operation: F.E.R.A.L..
- It is revealed that she is not as 'air-headed' as she leads everyone to believe, as seen in Operation: I.N.T.E.R.V.I.E.W.S., when Numbuh 1 is leaving and she asks him what they are doing the next day, to which he replies that he won't be there, and she says, "Oh, I know you're leaving, silly. What do you think I am, an airhead or something?"
- It is hinted that Numbuh 3 may be the one to create the acronyms for the missions and 2x4 tech as seen in The Grim Adventures of the KND, when the M.A.N.D.R.O.B.O.T. has a stupid acronym and Mandy asks Numbuh 3, "What kind of acronym is that?!", to which she replies, "Well, if you weren't in such a rush, I could've come up with a better one!"
- There is a running joke with Numbuh 3 where whenever she's ejected from Hippy-Hop, her parachute always comes out after she's already hit the ground.
- Kuki is the only member of Sector V to have never called Numbuh 1 "Nigie".
- Her given name is Japanese for "stem" (茎). It can also be written as 空気 (kūki), which means "air", and reflects her carefree personality.