How about theme weeks? She or you can plan some activities & set up a loose schedule for them. Does the 2 yr old still nap? I'd try to have some activities for all of them & some that involve 2 of them while the 3rd does something alone or separate.
These would have to be modified for age levels but even the little guy could have fun at his level w/older brothers to help.
Countries - pick countries, do some research, learn some foods, games, customs, words - maybe the older 2 could make some new foods while 2 yr old is sleeping? Or give 2 yr old playdoh to smash while they cook? Look at the geography on a map, figure out how to visit - how far away, find books about that country to read, etc.
Olympics - learn new games & make up some - balloon volleyball, obstacle course, goofy races, bean bags - they could make them then play; bubble catching, chalk long jump
Drama week - read plays, make up plays, put on performance & have costumes, make props, scenery - paint or color flattened boxes, etc.
Movie week - try new genres, learn backgrounds of characters or stories, read books related to the movies, color pics
Spy week - camo crafts, sofa cushions fort, have them make spy gear out of recyclables & tape, play hide & seek, set up scavenger hunt w/clues, there is a fun website about FBI for Kids somewhere online
Wars - research famous battles, make forts outside, have water balloon fights
Legos -different competitions - highest tower, most creative, best use of odd blocks, etc. create different mazes w/blocks - my 3rd graders loved to create Egyptian mazes from blocks & set up ****y traps & treasure for others to follow & figure out the maze
While most of these apply more to the older ones, the little one should be able to have fun too if his brothers will see themselves as the teachers, showing & helping him w/the fun stuff. Good luck!