They will destroy the grass pretty quickly (like days) if it's a small area so you may have to re-seed that once they are moved on. As to their happiness, that's up to you to judge. Some people are happy to keep 3 hens in a eglu all their lives, some are happy to keep them on cages in a shed, some give them lots of room and so on. It depends what you have to offer them really, only you can judge what is acceptable. If you have plans to move them to a bigger place then keeping them penned up over winter is no real hardship for them. As you say, give them lots to do, branches, perches, litter to scratch in. A dust bath (we use a cardboard box with wood ash in it as it kills lice too). Plus they love stuff like hanging up some spare greens like cabbage etc. Make it harder for them to get so it stimulates them. Good luck with them.