What does an enemy do when the target of its movement is on a space with 3 models?
If there are already three models on a node and another model moves onto that node, the players must push one of the three models already on the node.


The emphasized text above gives you your answer, the players themselves must decide who stays on the node and who is pushed off so as not to break the 3 model rule.

I would say this is enforced by the wording of "the players must" and the lack of listed priorities anywhere in the rule book for who should be pushed off first. It can be logically assumed that without an order provided, the players can decide an order for themselves.