Congrats, and I know what you mean about feeling more tired during a pregnancy you are older with! I could hardly do the stairs my last pregnancy

. (They are those old very steep stairs since our house is 100 yo, but STILL, I felt very out of shape.

) Anyway, getting a month of school under your belt before baby arrives is a great idea, and will do wonders to make you not feel behind when you do need to take the time off. I would spend real time helping your oldest get used to using the Bigger Hearts guide independently by having him read his plans from there and check off his boxes as he does them. He can get used to grabbing the appropriate books and setting out the needed materials for a box. I'm in the process of teaching my almost 7 to do this already. I have him call me when he's got his things ready for the next box. It seems like a little thing, but it saves a lot of time. I also would have him doing the boxes he is meant to do quite independently, independently. It's easy with just 1 to do more for them, but in preparation of having less time, I'd spend this next while really training your oldest to do as much on his own as possible, while still retaining your teaching time. Other than your teaching time, he can be responsible for the rest - materials, books, getting things out, putting things away, checking off completed boxes, turning in completed assignments - all of this can easily be done without your supervision, which will free up a LOT of time for you later.
After baby arrives and you've taken some time off for that, and after you've started Bigger Hearts back up with your oldest for a week or 2, I'd start 5.5 yo half-speed in LHFHG, with daily phonics. Blend him into your schedule and get a routine down with those two. For your almost 3 yo, I'd just read her some books before nap and bedtime first. Maybe pick an anthology to read slowly out loud to her, do a few fingerplays like Patty-cake, This Little Pig Went to Market, etc., but maybe wait to add LHTH for a month or so, until your older 2 are in a good routine already. Then, you can start LHTH half-speed with your 3 yo.
If this is overwhelming, just skip reading this next part! But, I had a schedule for baby, son doing LHFHG, and son doing Bigger Hearts I'll share here. It was very detailed as I needed that back then! Here goes:
Don't worry - you can completely do this!

Easing into it and stagger starting are 2 of my best plans when I've been pregnant or just had a baby. One small piece of advice, move forward, even if it is in bite-sized increments, and then pick up speed when you are able. You'll be very glad you did and though the pace may be slow at first, it is better than completely stopping for a longer period of time, as it just becomes harder to start back up, and then you begin to feel further behind. HTH!
In Christ,
Julie