Ahhhh ... the limit of three per thread makes perfect sense.  It's not a problem that goes away with a single "offender."  If you take away the limits, I'm sure there are current members who would take over the challenge and bury everybody else with sheer quantity. 

However, Helle makes a good point.  Sometimes there's a challenge that is so compelling, three doesn't seem adequate.  Perhaps spacing it out over the month?  If the challenge runs four weeks, maybe you could limit it to two or three per week per person.  That way if somebody has a lot to say, artistically speaking, they can come back later.  That might keep interest high throughout the entire month, too. 

Just an idea ... it might be too complicated in practice, but it's yours, for what it's worth!  :P

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