We're in the same position. I usually wait to potty train my kids until they're older. But, we might be doing a cruise next year, and I want the youngest to be potty trained beforehand. So, I'm potty training a 3 year old for the first time.
He's fully trained for pee, but he's only successfully pooped on the toilet once. I'll probably do bare butt with him again some time in November, but poop is just so much easier to "hold" and wait until we put a pull up on him for sleep. He typically wakes up dry, so maybe I'll skip pullups fully for a few days, and see if that gets him working.
We will be bringing and using pull ups. If he regresses, then so be it. Fortunately, the only public toilets he won't use are portapotties, so he should be fine at Disney. We'll just make him try when everyone else does.
Disney doesn't make it easy for kids to have accidents. The toilet stalls are just normal sized, and you'll need to have everything you need with you. Changing tables are there but not in higher numbers, except in the baby care centers. I think they do have changing tables in the mens room with more frequency than much of the rest of the US, but I'm not sure. Most of the bathrooms were fitted a long time ago with only minor updating over the years. The babycare centers are nice, but not particularly convenient. Even when I've gone with nursing children, I only use the baby care centers like once a day.
I won't make it harder on any of us. Heck, the 6 yo is still having almost daily pee accidents, because he doesn't go to the toilet until the pee starts coming out. We did a cross country road trip over the summer, and had to stop for him to pee practically every hour. I'm tempted to put a pull up on him at Disney. The 3 yo at least tells me when he has to go...
If you haven't already, I would make sure you're all comfortable with standing changes. Changing tables tend to be small, and 3 year olds are wiggly. It's just easier to find space to change a standing child than it is to find a place to lay them down.