Unless you need to do this to keep a nanny (i.e. it's a financial need) I would not. I nannyshare due to cost and it's been great for us and we do have 3 kids in the share but only occasionally does the nanny have all 3 due to school for oldest.

An infant is really limiting. The twins already have each other for socialization so the baby doesn't add anything from that angle. The baby is will be too young for a good while to be a nice extra playmate buddy for them. And nanny will now need to start working the schedule around naptime for all 3 which could mean she barely leaves the house. Strollers & car seat issues for 3 - particularly all that young - are not easy.

See zero benefit for you in this situation, OP.

I have been a nanny for 4 years and in those 4 years I have cared for 11 children from 8 families, but I mainly looked after 2 families long term who both have 3 children each all under the age of 7. At times I would have all 6 together, but they all survived.

It is definitely safe.

I honestly can't much advantage to you for this situation.
In fact, the only advantage I can see is financial.

Putting aside all the pitfalls - location, nap schedules, activities - that must be compromised on. Can you even imagine a way to be fair financially to all the players? THe nanny needs a significant raise. You should get a significant discount. Is the new family going to be willing to pay enough to cover both of those things (since they also should be getting a discount off of private nanny care)

Also, are your kids heading to preschool any time soon? You can't lower your nanny's wage when that happens. But I'd certainly expect to shift more of the cost to the infant family when that happens. Is that ok with them?

Personally, I think 2:1 nannyshare splits are very difficult (not impossible, but difficult) to work out to be fair and worthwhile to all parties.

Safe, yes if she is fine with three kids and can handle it. I would be more concerned about how the other mom is with naps and flexibility. At some point, your kids will be in activities and if baby is still on a two+ nap schedule, how will that impact your kids.

I'd expect something closer to 30-35. per hour.

Safe? Certainly.

The same attention? Not exactly. They won't be neglected or ignored, but there may be times when the other baby's needs might come first (just as there are times that one twin's needs will come before the other). I'm assuming since your nanny has been with you for 2 years, that your twins are 2+? Thats a big enough gap that they could potentially be siblings, so not so unfathomable.

As for pay, it's difficult to say since you don't mention her current wage. In that situation I think I personally would want a total of around $22-$25/hr. Something like a $15/$10 split seems about right.

Our Nanny has been working for us for 2 years now. We have two twin boys and they sometimes cry when she leaves, they love her.
A friend of ours has a newborn (7 months) and wants to share our nanny with us, which would then be 3 kids, my twin boys and her new born boy. The Nanny agreed to work with all 3 kids and I did too at first but now I am starting to ask myself if it is safe to care for 3 kids??? will my sons get the attention they used to get?? What should we pay her incase i considered it?
Thanks