Many games have suffered this fate (e.g., Game 1523340
Your best and only bet to end the game on your own terms is to manage to break both player's bonuses and have enough troops that you can defend your own, defeat their reinforcements, and out gain the two of them combined for the duration of the game. Unfortunately, that is not always an option once a game has progressed to a certain point.
In that case, you can use diplomacy and hope that somebody makes a mistake before you do. For example, you could offer the third place player a truce, and offer to bear a heavier burden to take out the second place player. You would basically offer to turn it in to a roughly equal 2-player game. This is pretty tricky to do, and if you want better than even odds of winning, you're probably going to have to be a bit sneaky.
You can suicide into one or both of them and move on with your life (and your other games). This is particularly useful if you don't care about the inevitable bad rating from one (or both) of the players.
Some people make a 3-player "tiebreaker" game, that has settings such that they believe it will actually end, and allow the winner of that game to win the original game, too. Sidenote for CC nerds: Based on what I read in very old forum posts, this is actually against the rules because it is technically game-throwing. The original game is thrown to the winner of the tiebreaker game. But this technical rule violation is ignored as long as it's used only when necessary
Or you can just keep playing and see who the last person alive (in RL) is and the rest of the players deadbeat
In the end, you don't have any really great options.
**Anyone know what happened to that old Midgard game (or whatever map) that had several thousands rounds to it, IIRC?