I've gone against a WH/EWH sniper that 3v1 on a node in Civil war and won. Cover pulse, flash bang, knock back on the heavy hitter, the defensive skills etc. My then mostly battlemaster assassin was destroyed before even getting close. Stealth and positioning didn't help at all. My other 2 battlemaster melee comrades were sent to the respawn quickly too. He picked us off one by one. Keep working on it and I'm sure you'll become better at it.
Stealth is a hard counter to snipers, how are you 3 so bad you got owned by one sniper. Did none of you have your cc break or def cds up at all? That's the only way I could possibly see him win.