With 3 doing it, it may be exhausting to take them back to their beds every time.
I'd do the sleeping bag/pillow on the floor (or blanket, old comforter, whatever you have). Put it there and they can crawl into it IF they do not wake you or crawl into your bed. If they get in your bed, they have one chance to get into the sleeping bag with no argument, otherwise it's back to their rooms. If they can sleep quietly on the floor with the security of having you nearby, fine. I wouldn't make an exception for "if you have a bad dream" or "if you don't feel well" because that will become the reason every time. (Obviously you would make an exception if there was a major fever but you don't have to tell them that now.)
Short term, it means you don't have any adult privacy, and I get that it's a problem. But your bed is your bed, and it's not a kid bed. Same as the wine rack or liquor cabinet or car keys or kitchen knives not being kid-allowed. Non-negotiable.
Longer term, the idea is that the floor will be uncomfortable and boring, compared to their nice warm beds with their special blankets and stuffed animals. If there's no interaction with you (no talking, no snuggling), the attraction will wear off. But still they will have the short-term security of not being alone.