At USPSA 3Gun matches they will not allow Suppressors, but as Jesse says, some Outlaw matches do. Our club does allow them in whatever division you shoot. We also have night matches and night vision is allowed in those too (as are lasers).
Really, if you want to give it a try, get some ammo and go. We have shooters show up with stuff you would never think of, even a bolt action .270 deer rifle last year. This year that same shooter has all upgraded gear since he found he liked it. In the competition shooting world I have found the 3Gun groups to be a great bunch.