Officers who searched the trio found individually packaged quantities of suspected class B drugs.
An eight-inch bladed kitchen knife was also found in a nearby hedge following a tip off from a member of the public.
A 16 year-old boy from Croydon was arrested on suspicion of supplying drugs and possession of an offensive weapon.
He will appear at Wimbledon Youth Court next month.
Two 16 year-old boys, also arrested, were released under investigation.