I agree with chico2 and 14+kitties, you should have more than one box. It is perfectly within your kitties rights not to want to share, and I wouldn't try to force the issue or you likely end up with some inappropriate eliminating. I'm not sure why your boxes are smelly if you have more than one. Do you clean them at least once a day (preferably twice if you have 3 cats)? What kind of litter do you use?
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler