S.L.
No pull ups at all. They make the child think he has a crutch and will go in them. Start with going whether he wants/needs to with a timer set for 15-30 min. Have him sit and if he goes fine and give him the reward, if not no reward but encouragement for next time. Reset timer. If he has an accident calmly clean it up and say next time we'll go in the potty. It's a job and with two more of a job. Maybe you should get one well on the way like you're doing before much pressure is put on the other child but I also know our twin grandsons are very competitive and that has helped them in many ways. I got a call last night from one saying he'd 'pooped' in the potty. He said, " Grandma, you will have to give me a big hug and a kiss." Which I will do. Tonight I got a call from the other one saying he'd gone too. They are three and have been potty trained but for some time now but having issues with the bowels. Competition is helping them get that down now. I have a 'toy reward' for them when this is done but their parents gave them a treat and stickers to start off with. I hope you stick with it, no pull ups and don't go back once you start.