I'll try to make this brief...
We live abroad right now. By chance our close friends from home have family near here, and the friends come and stay with their family for several weeks in the summer. The friends have kids around the same ages as ours and we spend a lot of time with them when they're here. We love our friends but they are quite messy/chaotic and their kids are like bulls in a china shop a bit boisterous, so it is quite full on to have them in our home.
We usually end up going on our family summer holiday during one of the weeks our friends are here. This year, in order to get the dates/location we wanted we had to hire a house that is quite a bit bigger than we need. It's a lovely beach house with a pool and lots of space.
We'd like to invite our friends to join us for part of the holiday. It would be a lot of fun but probably would get stressful after about three days, and we would like to have the rest of the week as family time. Our friends would be our guests and we don't expect them to pay for anything except their own transportation to the house, which is about a 6-hr drive or a short flight. They don't like driving so they are more likely to fly, which will be a considerable expense.
Our friends can afford to do this but understandably may think it's a lot to pay for such a short holiday. If so, they could extend it by sorting their own accommodation for additional time, or simply decline our invitation. It's not a summons after all 
. We just want to put it out there and will have no hard feelings if they don't come.
Our friends can be a bit tone-deaf so if we do this we need to be completely clear that we are only inviting them for three nights. Obviously they will know we have the house for a full week. We don't care which three nights they would come.
AIBU to invite them for only three nights and if so, how do I word the invitation?