I find that one of the most difficult positions to play from is being the leader, especially when there are only three players left. The other two are certain to ally against you, and you're certain to have the least experience at playing from the leader position (since in any game, you're more likely to lose than win).
What I do is take it slow: don't scare the other players into an alliance. Build up a non-threatening position of strength before going for the kill. I like to build up a reserve behind the front: even if they can see it in a sunny game, people tend to focus on their own game and don't consider your reserve a threat since it's behind the lines. But it allows you to completely commit your border forces at a time of your choosing, and then reinforce afterwards.
Another tactic I sometimes try is to deliberately allow the #2 player to become stronger than me (in troops per round), thereby causing #3 to attack them instead of me. That's a risky strategy because they can quickly take over if you miscalculate. And, of course, it's only viable if #3 is quite strong themselves.