I've seen thousands of portraits because of the work I do and I find I'm bothered by multiples in a portrait where all subjects are waist up and none have hands showing. I think if there is a chair involved, the 3 children can be positioned in such a way that you're getting a combination of waist (primarily) and 3/4, which I personally find more pleasing.
Take a look at Peggy Baumgaertner's portrait:
http://www.portraitartist.com/baumga.../charltons.htm
You might get some ideas at Hongmin Zou's site. He has a section on groups:
http://www.hongminzou.com
This one by Bart Lindstrom is only two children, but might give you some positioning ideas:
http://www.portraitartist.com/lindst...narosejohn.htm
Though these portraits might be larger than what you're planning, the positioning might still give you some ideas.