Well, yeah, but if you fire three times as many shots, you run out of ammunition three times as fast. Even if you have plenty of spare ammo, taking the time to reload is painful when you have a Reaper bearing down on you.
My rule of thumb is: if you have a high hit probability and you must kill an alien, then use burst fire. The classic case is when you rush an alien with a rifle armed Xenonaut. If you have no-one else who can take out the alien if your Xenonaut misses, then definitely use burst fire. Don't die wondering whether one more shot would have given you a hit, or a second hit would have given you a kill. But blazing away your ammunition from range with a 20% probability of hitting is usually pointless, and if you run out of ammo at a bad moment, can be positively dangerous.