There are two trains of thought here. Travel during the day and break at night or travel through the night and rest in day. Personally, I haven't preferred one over the other from my horses point of view, but 'I' prefer to travel during the day.
We would regularly travel between 3 and 5 hours to comps on weekends. A couple of times a year we would travel to a bigger comp 8 to 15 hours away. Sometimes, we would stay overnight somewhere and sometimes we wouldn't it just depended on where we were going and what was available.
We would stop every 3 hours or so and let the horses off to walk around, chew on some grass and drink (which they rarely did). We would do the same, get some food, walk around, shovel the poo out and then we'd be off again.
So if you are travelling 3 or so hours a day and your horses aren't used to travelling long distances, I'd have them off every hour or hour & a half to let them wander around and go to the toilet etc (some horses won't wee on the float). However, it really is governed by the horses, if they aren't travelling well or one decides to play up or whatever, then you'll obviously need to be flexible and chuck in a couple of extra stops.
I don't know about where you are, but here usually the showgrounds or racecourse is a good place to stop-over but you need to get permission first.
Only As Old As You Feel - Sometimes I Feel VERY Old