@0MammaBear0 you haven't said anything about how remotely you live - whether market town, village, or farmhouse with neighbours across the fields. Whichever it is though, presumably you knew this was the situation when you conceived (with other DC 3 & 1.5? is that right or did another poster pick that up from another thread of yours?)
I want to remain sympathetic as the run up to birth is stressful - but relying on MIL as your only option for first person to reach you for over a month is grossly unfair on her! She is entitled to a life & the freedom to have a day out with a friend, an hour shopping inside the big steel box at tescos where mobiles don't work well - whatever!
If you have two children that age and are a SAHM, you must have other friends, acquaintances & people living near you in your support network? (If not, what on earth were you thinking having a 3rd child under 5?!?) So NOW is the time to rally that whole network, regardless of whether MIL goes away or not.
It's basic risk management if you think you're going to drop that quickly.... if midwives won't be out under an hour, you can't get to hospital in under an hour, ambulance may take up to an hour, MIL lives 30 mins away and works, shops, and you know - has a life of her own - even she can't be your sole point of reliance for getting to you in time!
Map out - in time and location - who your options are... neighbours X are home M-F but erratically. Parent friend Y doesn't work Fridays. Parent friend Z doesn't work Tuesday & Thursdays. Old lady at the end of the village who always smiles at DC might be worth tapping up for a phone number so she could be emergency entertainment your 2 DC for an hour until next person arrives home to help etc. etc.
At the moment, you have all of your eggs in one basket with poor MIL - regardless of whether she has a holiday or not. What if her car breaks down day of labour? I'm not usually a fan of mumsnet what-if-ism.... but in this case, it's entirely reasonable to need and actively create a plan for alternatives that reaches far beyond just your MIL. And that will mean she can enjoy a well earned holiday before a glut of time supporting you, and be back, well rested, for when you are actually MOST likely to need her. And will probably still be the person that actually gets to you first in the overwhelmingly likely odds anyway!
I understand the DP bashing - it doesn't sound like this is the best thought through 3rd child if finances & his employment situation are precarious enough to not be able to take flex/unpaid leave to support this - but your wider network should be able to support the gaps with a bit of thought & extra work at developing your local connections.
You do sound like you're a long way out on a limb in terms of not much parent/friend network around you so far though, if MIL was your one & only plan to date? Work hard on building this wider support around you - and dare I say it, try to keep up a career option in the background as well. If DP lost his job or bailed in 12 months time, you are in quite a precarious position & need to work on all your other support & backup - beyond just his Mum. For many reasons!
Good luck with the birth, but do use these next 3 months to connect more deeply with your community around you - most people would be fine with knowing they are emergency option 5 of 8 on a list to collect your 2 DC for a few hours because of childbirth! And you'll have far better cover and peace of mind that way - as well as a much stronger support system around you for the coming 5 years til bump starts school.