I agree with PJ.  Limiting a thread like this to three submissions keeps things interesting.  Personally, I don't want to see 20 images by the same person - it's like being around somebody who just won't shut up.  They dominate the field, and it isn't fun for anybody else.  It's different in a running thread (like "The Picture Below Me," for example) because it's a dialog, and different people jump in and leave and come back, and it's interesting and fun.  I like the limit of three per thread for these challenges! 

"If your pictures aren't good enough, you're not close enough."  ...  Robert Capa