Basically, when that happens the isolated player will probably get knocked out, but his job is to make the other two players pay for it as dearly as possible.

Personally, I prefer to play on single star maps to avoid this, however.

I try to do what daynewagner does, but with not so much a fleet.

From my experience, most players are slow to go for Interstellar travel so long as there is an enemy in their star system. It's easy just from the start to tell where everyone is, and sending giant fleets to try to save your stranded player is usually out of the question. So the name of the game for them is economy and stomp the lone player (three or four players combined don't need to send full fleets each to neutralize a lone player).

So my goal is to get a Mothership out to the neutral system as soon as possible, and spam repair bays and whatever the best ship I can get at whatever the nearest choke point is (hopefully there is one).

Unless you're superman, your opponents are awful, or your opponents take a long time to get to you and you play Vasari and manage Dark Armada, your job isn't to beat who you're with, it's just to stall them. The longer it takes them to kill you, the more time your allies will have to beat them after you're dead and gone. If you're lucky and manage to take some places in the Neutral system, you can fortify it up somewhat and again stall them if they come for you, but with luck your allies will begin to travel out then as well. And then maybe you can witness their end, or if things go poorly, die alongside your allies rather than alone.

What a glorious life the lone player leads.

One of the important things to remember when fleeing is alegiance. If you flee to a new star your first colony will start at 25% allegiance. If you make that your homeworld it will now slowly start increasing to 100%. Every new planet u colonize now once u have a new home will be correct in relation to that new home planet.

A trick is to colonize one make it a home planet, colonize the next and the make it home then scuttle the first one and re colonize it, this obviously takes some cash but will alleviate a huge allegiance penalty which can really hurt you. Even if you cant do this make sure you save up enough to mark the first planet u colonize as homeworld and dont colonize anything else until it has been upgraded to home or you will cripple yourself with nasty allegiance rates.

This probably is most common in Foreign Invasion, as that is a 3v3 where one team member HAS to start isolated. On FI it's a bit easier to help out your team mate because he's not that far, but the enemies are still closer. The method that seems to work for me in the past, is having 1 player keep the enemy team member on your side busy/distracted and have the others go to attack/distract the enemy. This gives the isolated team member a chance to at least build up some defenses, and if you have pressure on their side while keeping their team member on YOUR side, you'll end up taking out at least one, maybe 2 of their team.

I hope that made sense