An au pair could be OK. They'd be able to do the mornings with the older two children so you wouldn't need to get up/dressed etc. can have a lie in/concentrate on the babies.
They'd be able to clear up the breakfast things, put a wash on, change the kids beds, pop out to the shop/drycleaners/post office if needed.
They wouldn't really be able to have sole charge of the newborns, but are perfectly capable of feeding/changing/dressing one while you do the other. They'd also be able to keep an eye on them/rock them so you get a chance for a shower etc. If they're the 'need to be walked to sleep' type then I'm sure an au pair can be sent out with them in the pram for a walk. Au pairs are not qualified/experienced childcarers, but as long as you get one with common sense they'd be perfectly capable of helping out with the babies.
They'd be able to entertain the 3 year old for the afternoon, take them to the park/library etc. They could do the afternoon school run, help with dinner and bath time.
An au pair would only normally work 25 hours a week, an au pair plus could do 35 hours, so they wouldn't be able to do everything I describe every day, but you could pick the hours you most need. An au pair would be the cheapest and most flexible type of help. Obviously you have to deal with someone else being part of the family/looking after another person etc. and some people do have really bad experiences with au pairs not being sensible/reliable/hardworking.
The other option would be an ex au pair wanting their first nanny job, newly qualified nanny, ex nursery worker etc. anyone looking to get into nannying, but who doesn't have the experience yet.