No experience with taking little ones but have done 4 x 4 night cruises, no way would I want to do 3 nights, you lose the 1st day boarding and unpacking, boat drill etc, last day you disembark 8-9am so your 3 days is really a 2 day cruise, too short IMO.
I am sure 3 days is better than none but if you have a choice 4 days would be my advise.