Hi Mandy. there might be several reasons your ex doesn’t want all three. He doesn’t enjoy the baby stage, he wants to take the older two out somewhere and taking a buggy/ baby makes that harder or less enjoyable. Or possibly if he’s been a bit controlling in the past, leaving the baby with you means you can’t have a evening out.
No-one can force a parent to parent IYSWIM. You could insist that he either has all three or none at all. He needs to accept that you need a break, but equally he could then refuse to have any of them. Or he may genuinely not feel confident with all three in which case he needs to practise. You can judge better than anyone.
But you have 16 years of co-parenting to get through so I think talking to him and getting to the bottom of it is essential.