Triple trailer 3500 axle's = 10,500 ........no problem ever trailering boat, but seems I'm just at capacity for boat matched to trailer.....should I be concerned?
If you have owned since new and never an issue what is prompting your question? It seems there is more to the story here. Perhaps just curiosity.
If so, and to be comfortable. go to your local sand / gravel quarry and weigh the boat / trailer. Just go talk to them prior to hauling up your boat. If not available, try the truck stop weigh stations. I have found the folks manning these stations to be very helpful. When weighing the boat have it loaded up as usual.
Another important aspect that I am sure you have considered but just in case is using a properly rated trailer tire. At 10,500 lbs of ‘rated axle capacity across 3 axles’ you have 10.500 / 6 tires = 1,750 lbs per tire. Ensure you tires are capable of this and frankly even more. I also set my tire pressure to the max for max load prior to hauling. Use a dial or digital gauge where the max is about 50% of the pressure you are checking (so if need 65# use a 100# gauge).
Another consideration for peace of mind is a protocol I follow when towing my travel trailer or boat: at every stop for gas or whatever, before I do anything else I walk completely around both the truck and trailer and feel the temperature of every tire and wheel hub along with checking the straps, hitch, etc. If a particular trailer tire (or hub) temp is noticeably warmer than the others, it’s a problem. After all these years, this is a habit and I do it without exception or fail.