Before you buy axles I would weigh the boat and trailer, if the boat has been used with that trailer for this many years it will be ok for a while longer or do you think that this isn't the original trailer?
My axles are at full capacity and haven't had any problems, I wouldn't worry to much especially for right now when you aren't going to be towing any long distances.
You could always just add a 4th axle it will make it harder to maneuver but would be the cheapest way to get it into capacity.
Give me a call,