3-of-a-kind bonus in Ruk Shuk
Disclaimer, I've never played this but found rules online and seem fairly straight forward.

Yes the 3-of-a-kind bonus can be score with 3 white rocks.   This has bee confirmed by designer on the BGG forums here

As far I as I can tell from BGG pictures of the game, all bridge rocks are grey, not white.  Also looking at rules here the bridge rocks are not even defined by a colour and just called 'bridge rock' as opposes to 'white rock', 'blue rock' etc so don't see why a bridge rock would could towards 3 white rocks.