So it sounds as if your 129 and 137 don't leave much room for a 3rd sled in a 20+5? Dealers and some here say an 18 + 5 is sufficient for 3 sleds. Maybe I'm not reading something right then.
What I am saying is that although my trailer is a 20 + 5, I really only use the 20' of straight box because I have used up most of the vee with shelves/cabinets/bench etc., so since I DO NOT stagger the sleds , I keep them directly inline, nose to tail they both use up of all of the 20'.
An 18+5 7' inline is considered by most to be a 3 place trailer, since you drive them in the front, the tail end ( narrowest part) of the third sled will fit nicely into the vee. I actually think I could get four ? into my 20 + 5 depending on how log they are, but there's plenty of room for three of just about any length in mine IF I used the vee space. Which I can do by just about a dozen screws and remove some of the stuff up front if I needed to.
By staggering we mean you drive the first sled in from the front, drive it all the way to the rear and over with the ski against one wall ( say drivers side ). Then drive the second sled in from the front, and put it against the other wall ( pass. side ) and pull it in as far as you can. Then pull the third sled in from the front and put it against the driver side wall and the tail of the third sled will fit into the vee nose.
If you really need to squeeze them in you can also kick the rear end of the sleds over to the wall to gain some more space so you can pull the next one up further into it.
Alot of it depends on how long each sled is and obviously how many sleds you are going to haul, but I think if you are going to haul three average size sleds and 18+5 will do the job. I got the 20 + 5 even knowing I will usually only haul two sleds, but since I don't want to bother with having to stagger them I needed the extra 2' , not to mention I'd rather have the two extra feet and not need it then need it and not have it. I actually got this trailer to set it up more for the motorcycles, because we haul alot more equipment to the race track with the bikes, but also just made sure it would be simple and easy to get the sleds in and out of it too.
For me it's just big enough without being too big.