Nobody can tell you what to do - everyone situation is so specific and you don’t have a crystal ball to see what the outcome would be.
your age isn’t an issue, 35 seemed to be pretty average to start having kids around my friend group.
if it was me it would be a hard no but that’s because I didn’t want 3, certainly didn’t want a 3rd after having my two, loved that easy 4-11yo stage, now very glad I don’t have a toddler to deal with along with all the complications that preteen and teen life have brought. I don’t think it would have been fair on my older two.
a very close friend did this and it has changed the dynamic hugely, to be honest even though her little one is lovely it’s not easy and often when we all hang out (her older two are the same age as my two) the toddler gets palmed off on grandparents as it’s just not fun for the teenagers to have to acquiesce to the little one’s schedule. They don’t want to have to play with him all the time, we’d just started being able to do more adventurous holidays with long hikes and late nights in restaurants altogether and now it’s all dictated by the younger child obviously needing more rest etc
but I do appreciate that it’s hard to say no to a gut a instinct, I would just not put any expectations on your other two to help in any way because that’s really not fair on them, and I guess they’ll want to be involved if there’s no pressure to, and I would have some solid childcare so you can still prioritise the older two on the odd day out or holiday