Ouch! You definitely need to try eliminating, or at least, crippling, one of them early on in the game. The TEC is unstoppable in late games, due to its impressive economic system. The best way to beat a TEC player is to cripple the economy. My favorite faction is TEC, and I know it pretty well. If I can stay on my feet early in the game, then I usually can wipe out anyone during a late game play. The Advent is pretty tough; I’ve played as them too. If your enemy is focusing on Javelis and Gardas, then you should pick close range sluggers like the Destra. Neither the Javelis nor the Garda is built for in-fighting.
Sovas are my all-time favorite capital ship because of their classic looks. However, they can also be pretty mean fighters later in the game. At no time should a Sova be taken lightly, however, if it is at all possible, eliminate the Sovas early on as they become insanely tough later in the game. The Advent carrier capital ship (Halcyon) is nice, and you should invest in it, however, make certain that you purchase some heavy brawlers. The Dunov might be able to duke it out with one, but the Sova never could up close. Basically, it sounds like you have a long-range, anti-fighter enemy. Focus on high hit points and sheer brute strength. Run your ships in close, and then rip them apart.
As for 3v1… Good luck! If these guys are known for teaming up, then you’ll probably have a hard time keeping them apart if they’re friends. If they are not friends, but only playing acquaintances, then you probably stand a better chance. I would focus on weakening one of them to the point where it would be a drag on the other two to support him. If you’re lucky, this might create some internal fighting between the three, and could result in a bit of natural selection. It would just be 2v1 then…
Basically, nail their economies. Disrupt their trade routes. Focus on firepower and longevity.
Darvin3 has great advice. The Rapture and Halcyon capital ships are excellent choices for this situation. Basically, you haven’t a prayer with three enemies, but you could probably manage two if you were careful. Divide and conquer. I personally like to use a fade-and-hit technique where I use two or three capital ships per fleet, and then use a bunch of frigates and cruisers to support them. I can make a half-dozen or more of these fleets, and I use them in pairs. I’ll have one trail the other a gravity well behind and use it as support. If I jump my aggressor fleet into a heavily defended well that I can’t win (even with both fleets), then I’ll jump out. If I’m lucky, then I can also get my enemy fleet to pursue me right back into the jaws of my second fleet. This is a nice way to whittle down the odds and make gravity well easier to take. Also, don’t position a strong fleet on your fringe worlds. If you keep a light one on the rim world, and it’s scouted, then you can fool your enemy into thinking that it’s lightly defended. I favor a strong fleet conspicuous on one side of my empire, and a light fleet on the other. If I think that my light fleet’s world is being scouted for attack, then I’ll often make a thrust on the other side of the map. This makes it look as though my focus is on one side of my empire, and can spur an attack on what appears to be a weakly defended world. Once the enemy has jumped into that world’s gravity well and actively engaged my defenders, then I jump my reserve fleet into the well and proceed to shred him.
Basically, just look for ways to narrow the odds.