My lo was in nursery 4 days but I've just upped to 5 in preparation for the baby. She's just turned 3. When I had her my husband was working part-time from home and I had lots of help. He's just started teaching full-time so I think I'll find it a lot harder this time. Anyway, if you can I'd keep him in 3 days to maintain his routine and he'll still have lots of time to spend with the newborn. I feel the same as @Charlie.
Keeping my almost 3yo in 4 days a week, then dropping to 3 days in the new year once I have a handle on having 2 little ones. My son thrives at nursery, so figured it benefits everyone :)
Our little one will still go and will hopefully start 3 full days. He's 4 and is my step son who is with us full time usually. I know I will benefit from the break of having both and it's not been a consideration to drop a day or any of it. He absolutely loves "school" as he calls it and runs in in the mornings. He has great experiences and gets those days with peers and makes friends. He will get far more there than I can and will be able to provide for him at home breastfeeding a newborn! They will still bond, also it will give them their own space that isn't taken up by a newborn that demands and needs a lot of the attention they are used to. It's a big transition and their own routine will help I think!
My 2.5yo son is still going to daycare while I or my husband is off work with the NB. Figured it is easier for us and he still gets to bond on the weekend. He loves daycare and spending time with friends, I also feel they can offer him more in the way of activities than I can normally, let alone while looking after a newborn.