Assuming 100 deep and both players are regulars, 3betting range of a BU against an UTG open has almost no bluffs as bluffing a tight range is unprofitable. Most regs will 3bet KK+, some throw in QQ, and a few will add AK. If the reg appears to be tight, it's far more likely to be KK+.

Above is all based off of my very limited experience. Above is also all very villain dependent. I think we are in agreement that to really talk about this we should have a HH infront of us.

As for the definition of "very tight" I'd say anyone with a VPIP under 16.