Dear L.,
Wow! Triplets! You have your hands full!
What a blessing though.
It sounds like they are pretty secure in their routines which is awesome. Each one knows their own bed and where they belong. That's not such a bad thing. But, as they get older, they will need to sleep in different places with different friends for sleepovers or if you want to go out of town to stay with family or go camping.
My first thought was to get them each their own little sleeping bags. My kids loved their sleeping bags. They wanted to lay on the floor and watch TV in them and if we travelled, they had their own bags and were used to them so it didn't matter where we went as long as they had them. They make really darling little sleeping bags. Blues Clues, Dora, Barbie...whatever your kids are into. But, having three the same age.....
Your kids might be a little young yet, but my kids loved sleeping bags so much that when they got a little older, I just bought inexpensive plain ones. I let them choose their color. Then, we used magic markers to write their names on them and I let them decorate their own. Smiley faces, moon and stars, flowers, the outline of their own hands. You can try fabric paints also, but they don't seem to hold up through washings as well. My daughter was so into her sleeping bag that we bought her a new one for 6th grade camp.....away from mom for a whole week. We decorated her sleeping bag. It was the cutest thing you've ever seen. And there was no issue with getting it mixed up with anyone elses. My daughter is 22 and still has it.
Just get creative. When it's naptime or nice outside, just let them chill in their sleeping bags on the living room floor or out in the yard after a picnic. Supervised, of course.
Give it a whirl and best of wishes!