Reinforcing the frame around the axles would be a very good idea; ask me how I know! ;)
From what I have discovered, our trailers have 8k lb axles (they are listed in the brochure as 7k lbs but it doesn’t appear that Dexter made the Never Lube axles with disc brakes at that rating) so the third axle may not be necessary. Getting it to look good will also require a lot of cosmetic fibreglass skirt work which would likely end up being very pricey.
I would think you would want to add several heavy crossmembers between the frame rails behind the axles too. As much as the c channel frame is large, it is quite thin material that is prone to a lot of flex.