Welcome to Mirage Enterprises!
There are obvious advantages to equipping your trailer with either tandem or triple axles.
One of the very first things to do when shopping for a trailer is to honestly evaluate how you intend to use your purchase. This is especially important when buying a trailer to haul your sleds.
When most manufacturers list a GVW (Gross Vehicle Weight) they are telling you the standard approximate weight of the trailer not including additional options (which could substantially increase this figure). Most of us know that to figure what can safely be hauled, one must take the GVW and subtract the weight of what is being carried. Then apply that to what your axles are rated. A tandem, 5200# snowmobile trailer starts with 10,000 pounds and you subtract from there. Do you have a fuel system? Propane Tanks? Heavy cabinets? Heaters? What do your sleds weigh full of fuel, or after riding in snow and ice? There are endless variables. For a small fee, you can stop at public scale and find out what you have loaded on your axles.
One of the main advantages to adding the third axle is the obvious increase in capacity; usually 45-65%. You will appreciably increase your braking ability as well as strengthen the structural capabilities. Some people feel that the trailer will tow better with three axles, and should you suffer a blowout on a tire, you will still have tires left to carry the load till you can pull off the roadway.
There is of course, a downside; the obvious being the additional cost. The trailer will weigh more empty and there are more tires and axles to maintain. There will be slightly more wear on the sidewall of the tires and most axle manufacturers will refuse to warranty “torsion” triple-axle configurations.
The most important factor to consider when making this decision is safety. Honestly evaluate your usage and needs and proceed accordingly.