Op, I think I'm in a fairly similar position to you, although in a lower paid career; my DH is self employed and I work FT and am the main breadwinner. We have fairly recently had our third child and, so far, it was absolutely the right decision for us. Yes it's a new level of juggling and it is very tiring, but my older 2 DC absolutely adore their young sibling and I'm so glad we have the third addition. We have age gaps of around 3y with each and it's worked very well for us. We use nursery childcare, rather than having a nanny, because that's what our finances permit.
Some things that have helped us to manage, without a nanny:
•DH being self employed helps as he's flexible - however we also have to be aware of the fact that; if he doesn't work, he doesn't get paid!
•DH does drop off every day, and collects on 2 days
•I work FT but have adjusted my hours so that I can collect on 3 days - I start very early and finish early on these 3 days and then work long days on the other 2 to make up for it. On the days I collect DH works longer and won't always be home before the kids are in bed
•Currently I work from home on 3 days (the shorter days) but as things are going back to normal my manager wants this to reduce (I'm not happy about this as it makes logistics much easier when I'm closer to school, but I may just have to suck it up)
•Our eldest has cut down on activities/clubs since DC3 arrived
Some things to consider:
•We had to change our car in order to fit 3 car seats across the back
•We will probably need to move house at some point, currently we live on the outskirts of London and are considering moving to another part of the country where it's cheaper. This will be a hard choice, if we do it, as all our family and friends are around here. But we are unlikely to be able to afford a 4 bed here. The older DC are sharing a bedroom until we decide what to do, but we're all a bit crammed in and uncomfortable, space wise, currently.
•I had been hoping to work towards a PhD, when we decided to have DC3 part of my decision making included accepting that I may have to let this dream go. I'm not saying it's not possible, but I'm not devoting a lot of time and energy to trying to secure this which has freed up some of my mental capacity.
•Our shifted working patterns mean we both spend a lot of time parenting the DC on our own. This works for us because it allows us to have time with the DC, but it is tiring and we do both miss the others input at times. We work very well together as a team and have a very close relationship and I do miss that time with him, however I'm aware that this period of time is limited and, for us, we've decided it's worth it to have a close relationship with our children.
Overall it's been the right decision for us, and my eldest often comes up to me and says how pleased they are with their youngest sibling. "Thank you for having another baby mummy, I love them SO much" which makes all the hard bits so worthwhile. It's definitely not easy, but it's doable.