Can you post some photos? That might help with identifying the problem. Might be a bit hard to say why those holes are in there, but that would have let fungus in. Normally fungus wouldn't be able to get in that deep unless there was some sort of injury to the trunk. By the way, Foxtails aren't multi-trunking palms, you had 3 separate plants in that pot. Most probably the smaller one was too weak and was choked out by the stronger ones.