As I mentioned in your other thread, I have a 6’ wide 2H stock/combo. Mine are 16.1h, 15.1h, and 12h. While I have never tried, I have no doubt I could fit all 3 in there in a total emergency, but my smallest is smaller than yours.
But a few thoughts:
I said this in your other thread, but a 6’ wide slant can be limited in it’s capacity as a slant load. A tip I learned (after I bought my trailer) is a good way to get an idea if your horses will fit in the slant stalls is to measure the length of the divider, then measure your horses from nose to tail. If the divider is longer than your horse, they should fit. If the divider is shorter than your horse, there are no guarantees. Brands differ in their length of divider, even if they are all the same floor dimensions, because you can configure the angle of the slant in different ways.
When I bought my trailer, I planned on using it as an open box for my 16.1h horse because I knew the stalls were small for her… but I foolishly assumed my 15.1h horse would fit in the front slant stall. Nope. Although for what it’s worth, my 15.1h horse is long and lanky; the 16.1h horse has a more upright conformation and almost fits in the front slant stall while I can’t even close the divider on the 15.1h horse, go figure.
If you decide to go with an open stock trailer instead of the stock/combo, I suggest up-sizing to a 16’ length (regardless of width). I think your horses would probably all fit in the 14’… but… if you want one with a gate to divide it into 2 stalls, there’s no good location for it to accommodate horses in the 14’ length. Some manufacturers put it in the center, leaving you with two awkward 7’ length stalls. More commonly, they place the gate off center so you have a 6’ long stall in the front and an 8’ long stall in the back. The front is too small for anything but ponies, but using only the back stall puts all the weight behind the axles. Of course, you could get a 14’ without a gate and eliminate this problem entirely…
Lastly, I don’t think anyone regrets buying a bigger trailer (within reason). If you have a wider option, you will appreciate it. Especially since you’re only talking about a difference of +/- 9" max which will be barely noticeable driving.