How big is the run that you mention was attached to that hutch?
If they have a run, say 4x4 attached, I'd say that was okay for just 3 days.
If you dont have the run, then I wouldnt even consider it personally. And I think the reason that you are feeling guilty is because you know it isnt really big enough. My lionhead is 1.7kg and a bit fat, so rabbit size wise she's probably a similar size to your mini-lops, and there is no way I would put 2 of her in a 3ft hutch. My hamster cage was only 10cm smaller than that hutch, and it was only minimum rspca size. I know even my little lionhead on her own would be frustrated and stressed in it and she is really good with being confined.
It is easily solved though. Can you not give your parents your 4ft hutch and keep the 3ft one for the shed? They have the shed 24/7 anyway so only need a snuggly area. Or you could buy a run, even if its a foldaway one, to attach to the hutch so your buns can have 24 hour access to it. Or just buy a bigger setup.
Remember that, even though you may be turfing out the rescue buns, by boarding there you are allowing the rescue to function, without boarders that bunny may not ever have been rescued in the first place.