My first Master-Cleanse i had planned to go as long as i could (10 days maybe) but due to circumstances, i had to stop after 3.
Day 2 was the roughest due to caffeine withdrawl.
by day 4, the day i broke, i felt fabulous... i took 2 days off (1 for breaking and 1 for a Bday dinner) then i hopped back on for another 4.
I think if your diet isn't that bad, not a sugar/caffeine junkie (sugar withdrawals have to be the WORST) - a 3 day wouldnt be that bad, and you'd get some benefit from it.
All of my MC's have been short ones, between 3 and 5 days.... just when you finish up, keep your diet clean and keep up on the water intake.