I will probably do the 3 day method to get my other two trained. One DD is basically trained. Sometimes she asks to wear a diaper but so far this weekend has been in undies all the time and no accidents. M & Ms totally work for her.
Reporting back to say it appears to have worked for my singleton. Today is day 3 and we are successfully using the potty for pee and poop. He is still a little hesitant with poop so we have to watch him closely when he needs to go and keep running him back to the potty. But i think I may only have two kids in diapers now!!! Well aside from bed, but still.
But,I can see this being much harder with twins. The first day there was a lot of false starts where he started peeing and then said, I'm peeing and we had to run him to the potty. Granted we are also taking care of 5 mo old twins at the same time so it was complicated lol. But the amount of pee with two will be a lot!! I'll try it again with the girls though because it does seem to work.
We did it about a month after our boys turned 3. It was an intense weekend (and on Day 3, DH had to work a 14-hr day so I was on my own) but it worked well. I recommend it.