I don't think it's anything you are doing wrong... But you being pregnant and your obvious struggle with his sleep is probably part of the problem. Some children are very in tune with the emotional energy around them from a very young age and when what they do is perceived as up settings their parents they pick up on that and it unsettles them and lots sleep time is when the insecurity comes out.
It definitely comes out that at almost 3 he's harder work than your 5yo and that's what's totally normal 3yo are super trying, they NEED connection yet want independence.. their independence frightens them and can cause regression with sleep too, because they are alone.
Unfortunately the problem is he needs what you also need to give to your NB, he needs you to step away from being firm with him and the routine and give him extra nurturing and trust that you are there for him.
At around 3yo both my boys had a camp out spot on my bedroom floor, it was just a folded quit with a sleeping bag and pillow... Not too comfortable, but a safe place when they needed our comfort...both got to the point they didn't even wake me when they came in the room.
One of the most important things I learnt was to focus on theost important need...a good night sleep...it's not a child sleeping in their room at all costs...it's that everyone in the family gets the best possible sleep they can AND with a newborn and 'the middle child' his need to feel safe at night is an important part of your solution.
