What will happen if I feed this rangefinder 3.3v?
You shouldn't have any problems with two regulators. You really aren't putting the two in parallel. Both inputs will be connected to the voltage source and the differnt voltage outputs will go to the different sections of the circuit. (You still need your input and output caps). I do this quite frequently with 12V and 5V regulators without any problems.

I didn't see a minimum trigger voltage spec for the TTL trigger pulse going to the range finder so you'll need to ensure that it will recognise the signals from your 3V MCU as high.

At the worst, you would need a logic level converter like this one from Sparkfun.