i have a avic 90 bt and i have been reading that some guys grounds the 3 grounds to the same bolt, and others say ground it to three diffrent bolts. would it make a difference if i grounded all three grounds to the same bolt? id rather do it that way.

 

As long as that 1 bolt is a good ground, it makes no difference.

 

And by "good ground" it just needs to be clean from paint or other contaminants and nice and tight. You can take a razor blade and scratch the area around the bolt to ensure a good connection but generally behind the dash it will be fine as is.

 

There's a factory grounding point behind the triple gauge that is used for many of the dash instruments. The best ground would run all the way back to the battery itself.