organization is the key. i like the Elfa system which in the states you can get in the container store. also, ikea has good options. you can talk with them about your needs and they can help you configure it to those needs.
now, i think that 3 in a room is great and perfectly normal. i have been in homes where families have 4 in a room smaller than that. two sets of bunk beds, closet the normal size, and so they used the elfa system. a set of wire drawers for each, and then they could use the pole above for their hanging clothes, toys were kept in another space. books for night-time reading were kept in bags that hung on the wall next to their beds. it was a very cool room.
in scandinavia, friends of ours live in 400 sq ft with 4 children and 2 adults. there is only one bedroom in the home, btw. so the other "main room" functions as the parent's bedroom, living room and dining room. the children share the bedroom, which is much narrower than the room you describe--probably 10x12 or 10x10.
anyway, again, two sets of bunks. no closet because it's an old building. so, on either side of the bedroom door were two 'wardrobes.' each wardrobe was divided in half for each child's clothes, shoes, etc. then, under the bunks were two boxes each--this is where the children stored their toys. between the bunks and the wardrobe were two book cases--one on each wall, opposite each other. these contained the children's books.
the main room was the parents room as well as the main living space. That room was about 12 x 15 or so. when you walked through the doorway, you were facing a bay window. on either side of the bay window were books shelves and there was also a window seat with storage drawers beneath. on the far end of the room, there were more bookshelves with open and closed shelves. that would be the 'left side' of the room. there, you would find the sofa with hide away bed (where the parents slept), a small coffee table with storage beneath (drawers again), and a chair that also had a small hide away bed for guests.
on the right side of the room was a narrow dining table and two benches--one against the right-side wall, the other between the table and the 'living room' area. the sofa in that area touched the "back" wall where the door was. the book shelves on the 'dining room' side of the room had dishes and various things for dining. the shelves in the living room side had books, tv, baskets of small toys, dvds, etc. the wardrobes on the far left wall had the parent's clothes, linens, etc. and the storage under the window seat housed seasonal items and decorations.
for a tiny place, for six people, it was amazingly organized, neat and tidy.
so, it can be done!