Play room or sleeping room?

If this is where they will sleep then I would get a twin over double bunk bed and a ceiling mounted curtain or retractable screen and call it a day. It would hardly be a really nice space to spend a lot of time as it’s open to elsewhere. That said in tons of vacation rentals there are lofts with multi person sleeping spaces

If it’s more like a play space then the furniture in the layout is actually not bad if you replace the desk with a comfy chair.

Bedroom ;)

I was going to say bunk bed until I noticed the High bunk would possibly be next to the open stair well !! That would be so dangerous. !!!!! Not enough room for the bunk bed on the desk wall. Maybe the 3 kids should sleep in your room when they visit and you sleep in the den on a pull out sofa? If you are determined to have the kids sleep in the den, let each kid pick out a super comfy sleeping bag and put each bag over a foam cushion on the floor. Then do really fun, active things during the day so they conk out fast at night.

There is totally enough room on the wall that sofa and desk are on.

If you were worried about safety with glass wall you could enclose that wall or extend the glass to ceiling but I doubt it’s necessary.

Think about a bunk bed with a trundle under the bottom bunk. Twelve year old on top, 10 and 5 on the bottom two. Place it where the desk is. Not sure how private it can be with the glass and the open area behind the person in photo #3.

get a 3 tier bunk. you could get fancy and get beds that pull down from the wall so they can be put away.

Add three IKEA chair beds. Then most of the time you can use it as a reading/sitting area. Convert it when the boys are over.

https://www.ikea.com/us/en/cat/chair-beds-16296/

It looks like it is about 6 feet by 8 feet (desk and sofa area). I can't see any way to make this open space a "functional, private, welcoming place for a three kids‘ bedroom." The stairwell would have to be enclosed for safety. Then you have the problem of squeezing three kids (boys? girls?) beds into a 6 foot by 8 foot space which does not even have a window. Even in New York City, that's too small for a legal bedroom (see https://www.brickunderground.com/rent/brictionary-what-is-a-legal-bedroom-in-nyc). I suggest looking around for alternative space. Sleeping bags on fold out foam in the living room would be something to consider.

Isn’t this a temporary guest type thing? Not an ongoing bedroom? Of course you can set up occasional space in a loft. Check out any Airbnb rental and they are endless. Heck we are going to one next week that is one bed and loft where kids will sleep!