If your lover is pregnant then that opens you up to an adultery Petition. That makes no difference to the asset split of the divorce, but it does invite a costs claim that would be virtually impossible to defend. If I were you then, if a divorce is not already in progress, I would file one quick smart.
Yes having another child in your new household will reduce the child maintenance you pay for the first 2 - not by a huge amount.
It doesn''t matter that you didn''t start the relationship till after separation, you did start it while still married. Therefore it counts as adultery and so, by confirming your partner is now pregnant, you are admitting to that adultery.
Generally if someone petitions on the grounds of adultery and claims the costs of raising that divorce from the respondent, they are successful.
So the suggestion is that you act first, Petition your Ex on the grounds of unreasonable behaviour. That way you might be able to avoid costs or at least agree to split them.