Been playing a bit with 4 sigmas just because they're a lot of fun. Here's a list using three:
3 sigma squadron (75)
1 Omicron with Vader and Tactical Jammer (25)
Usually when I play Quad Sigma I lose a ship in the first round to concentrated fire. (I have no problem with this because it usually means I take dow at least 25 points of my opponents list) Maybe adding a shuttle to protect the three with vader to clean up if needed will be helpful.