Anyone would love an extra room being added to their home. A little more of any good thing is always useful and wanted. But, if the room only allows three items, I am not placing anything (permanently) in that room at all. I would use it as an all purpose room and call it the “3 things” room.
I would carry a mat or a carpet, a simple source of music and do yoga, aerobics, dance or simply meditation.
Take a paint brush and colours to paint things on the walls.
A lounge chair, a book, and a drink.
And so on…
Oh, in case humans do not count as items all you need is a decent sound system and a bunch of happy people…and paaarty!! (Yeah. no food or drinks. I’m allowed only 3 items)