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.
Three assassins/melee should always be able to destroy one sniper even if he is max geared. Only way they would have trouble is if all three were in Recruit or the sniper had a healer behind him as they so often do (at least if I'm playing my healer
). I don't know what went wrong with that picture.