Is it legal to use a SBR in a 3 gun match? A silencer?
It really all depends on the rules of the game you are playing. I have used various NFA stuff at different pistol & rifle matches. Depending on the type of match and the setup, they can be an advantage, a disadvantage, or no difference.
A silencer acts like a really big muzzle brake, which is good. Some matches may treat this as a brake, and put you in the appropriate category. I want to say that there was a ruling in USPSA 3-gun that a suppressor does not necessarily count as a brake, so you could still shoot in Lim or Tactical where there are limits to the physical size of a brake.
I've used subguns in semi-auto with good results shooting pistol-caliber carbine matches.
Down-side to using a rifle-caliber SBR is your reduced muzzle velocity. You may have to shoot relatively heavy bullets to make a given power factor. Also, when shooting suppressed the timer may not pick up your shots.
You didn't ask about shooting suppressors on a pistol. I've done that a few times too. If anything, it's a disadvantage. The booster gives you a weird 2-stage feel to the recoil. It was really noticeable when shooting my USPSA race gun, and then the suppressed Glock.
Bottom line is that you can use whatever equipment the rules allow. Practice with whatever equipment you plan to shoot with. If you want to use some of your NFA stuff "just because", there's nothing wrong with that either. Just be safe and have fun.