Hi OP. Of course you're not unreasonable to expect half of everything - including the assets he brought into the marriage.
Children change everything. Of course, your salary progression will have been affected in a way his has not. And if course, you probably take on more of the 'mental load' than he does. I'm not saying he's a disconnected father, but this is just how it evolves with kids.
Put it this way OP, I've been a SAHM for 17 years and so obviously, it's my husbands salary that has bought the properties, school fees and everything. But he wanted a family; he is not a single man and he could only make the money he has because he didn't have to think or worry about anything else. We also live in an expensive part of London where a studio flat over a shop would be £500k. But if we divorced, I'd receive half of everything, plus the DC are now teens and DH has sorted them with trust funds and other investments for their futures.
Of course it wouldn't be reasonable to expect me to move to a 'cheaper area.' The schools and our lives and friends have been here for almost 20 years. The same applies to you and anyone whose living situation comes to be determined by a higher-earning spouse. Surely your DH wouldn't want his kids to have to move away?
I would be very suspicious that your DH tried to effectively 'cut you out' of his £3m when you got married. That's bad enough. But now that you have children, this 'my money, your money' ship should have sailed surely? The question is, why does he not WANT to share with you - as the mother of his children? Yes, you probably could get a flat in your area on your salary, but this is not the point, as you say. Would he really have you and the kids living in a flat, while he stays in a house?
To summarise, you are married with kids. What's his is yours and vice versa. Him trying to ring fence 'his' money will chip away at your relationship over time. Fgs, this is not a celebrity marriage where some 70 year-old multi-millionaire has married some 20-something who is obviously only after one thing. You both earn well - it's not as if you're a SAHM like me! He should actively want to give you security and you shouldn't need to ask. How dare he gaslight you and make you doubt yourself in this way.