If you are avoiding crowds, I'd say Halloweeen is probably the lightest of those three. Not too much happening at parks (other than nighttime Halloween party at MK which happens for a few weeks, which is a whole other ticketed event (and could actually lighten up the daytime crowds on days the Halloween party is happening). The whole of October is not peak time at all, and kids don't get time off from school for that holiday, and there really isn't a drive to come to a theme park for it.
Easter week or Memorial day I would expect to be horrendous, as kids get time off from school, and whenever kids are off from school, expect peak crowds at Disney as people book their vacations during those breaks. That is just general rule of thumb.