Get a solicitor.
Find out your legal and financial rights from them, do not listen to anything he may try to make you believe about they may or may not be.
Do not let him tell you 'we don't need a solicitor, we can sort this out ourselves.'
He is not your friend.
You are married - you are joint owners of ALL the assets including the house. Start from that premise.
Get the finances familiar and under control.
Assume there is probably an OW. One who is not preoccupied with the needs of three children under 4. One who puts him at the centre of the universe - which in his mind is the correct arrangement. One who reminds him of how things used to be pre responsibilities. They don't jump out of the family without a parachute.
As for the arguments etc, they do this too; rewriting history to make themselves feel in the right. The alibi, the justification.
The law will not care, though. Only about property, money and children.
He has responsibilities, 50/50. I doubt he will want that cramping his style, or alternatively he may threaten to go for that, if you get awkward.
This is a) to avoid paying maintenance and b) to control and intimidate you into compliance.
You need solid advice asap.