Best way to win against 2 opponents is to hit 'em early, hit 'em hard, and hit 'em fast.  What you want to do is try to prevent them from getting their economies up and rolling.  If your enemies succeed in doing that, you run the risk of getting slowly ground down in a war of attrition.

If your enemies do get built up and running, the best thing to do is defend against one and attack the other.  Use a small reserve fleet, as you already are, and park it on a chokepoint world.  Fortify the chokepoint with fixed defenses.  This will allow you to use a smaller mobile force to defend that world, freeing up ships for offensive operations against the other AI.  Also, you have to aggressively use scout ships.  In this situation, you have to achieve an impressive kill / loss ratio in order to win.  To do that, you'll need to know what ships the enemy is using so you can build ships that counter the opposing fleet.  If the enemy is using hordes of carriers, build your own carriers plus flak.  If he's using lots of LRFs, drop a horde of heavy cruisers on him plus some carriers with interceptors.

Also, since this is a long game, consider using the Kostura cannon.  You fire the cannon at enemy planets, and the projectile will disable all enemy ships in orbit for a short period.  Time your fleet's phase jump just right and you get a free turkey shoot for a little bit.

Also make use of Vasari subverter cruisers with the Distortion Field upgrade.  You can seriously harm numerically superior opponents with them.

The Dunov's Shield Restore doesn't increase shield mitigation. Any and all shield mitigation bonuses belong to the Advent.

Never try this again human opponents since you'd get your butt fried....but against an AI, you can defend your planets pretty well with just defensive platforms/repair facilities and a few ships. That'll be your key defense against one of the AI while you go and flatten another one.

Early on, AI doesn't defend his colony and builders too well. If you build bunch of scouts (aound 6) and send em out, you migt be able to kill one of the AI's colony ship before he can colonize. You can do this repeatedly (and run away when you encounter his defensive fleet).

AI have this tendency to build a mass of LF if you take out his builders initially, so tech-rushing to LRM and massing up early on also helps. AI also tends to overbuild on capitals, so they should be an easy way of hitting em hard while you suffer minimal loss.

Go for a minimalistic approach to doing research. Do you really need those LRM damage research early on? Of course not; skip those damage bonus research and go straight for hoshiko and carriers, and later on kodiaks. Also, plan your research as you go. Don't build too many labs if you can't afford to research AND use the researched ability. For example, researching hoshiko early on while your fleet only consist of several LRMs doesn't really make sense.

Another thing to note is that AI doesn't really know how to rush properly, so you can do a hybrid tech-econ rush early on which would be suicidal against human opponents. While you keep your initial fleet to a minimal, build two civic research and get a trade port. That should allow you to kick your econ to gear while keeping the number of planets you have to colonize and defend to a minimal.

That just about covers it. Best of lucks.

Play against humans, like me and no AI will seem hard anymore...