When each of my youngest 3 turned 3 (approximately) we threw a 'Pants Party'. They each knew about their party well in advance and the younger ones heard the tales of the previous glorious occasions when the older siblings had their parties.
The whole extended family came to each party, bearing gifts of underpants and even a pants-shaped cake. We had lots of yummy food, music, games, and other exciting events and at one point in the proceedings the child of honour would make a speech, "I'm not a baby anymore."
They each fondly remember their pants party and they were each out of nappies that day.