HI Pippin. Firstly, when I read your post, I totally understood how you must feel. I had a 23 month age gap and remember just how challenging it was! I am pg now, and although my other two are a bit older, I think I would find it hard to have two under three whilst pg!
The other thing is that my two behaved just like yours. My son's behaviour def got worse when he started pre-school and then school. It was a temporary phase where he was finding his feet with the other kids and new expectations. However, it's always worth checking if there is anything else going on at the setting/elsewhere that might be upsetting him.
Also re the attention seeking and aggression - there are things you can do to improve the situation and make things a bit more manageable for yourself. 'Cause three under three is tough! That's probably why you've had some reactions. But lots of people do it, and it all works out somehow. (And it's brilliant when you see them playing together and having fun for two hours without any adult support!! That's the stage mine are at. So hang in there!) My two both went through a hair pulling, biting, hitting stage. They don't get empathy until at least four, but you can talk about kind hands, and kind feet and give an immediate 'No'. I found that this works best when you then look for something you can then immediately praise. Somehow this just turns the behaviour around. This all worked for me, but took a while to bed in.
I found getting out of the house was always easier than staying in! Somehow their behaviour was always better.
The other thing that helped my friend, which I still kick myself for not doing, was to keep the routine of an afternoon nap for the two youngest. If your son is home in the pm I would use some of this time to give him lots of special attention. 20 mins of 1:1 can have major impact. The trick is to keep talking about it as his special time, and he will have special time with you every day etc. I used to involve my eldest in housework. They loved unloading the w-machine and turning it on for me.
Make sure he gets the chance to tell about his pre-school sessions and gets lots of praise/interest. (So he feels grown up rather than missing out on other stuff with the other two etc). Having this def helps to minimise jealousy.
The other thing I did which worked was to buy a small gift from the baby to the younger siblings for when they visit in hopsital or the baby comes home. My eldest still talks about this!
If there is anyone who can step in and help out, see if you can get that arranged now. Even if it's to play with one child for an hour, or to take them to the park. We all need a breather sometimes!
Good luck.