Tbh 3-5 is as good an age as you can get for these things to happen. They grow up with it being very normal for their parents living separately and in the case of a 3 year old will likely not actually remember parents living together after a little while, unlike say a 12 year old who will have more memories of living with both parents and more cognisant of what's going on. 'This is our house now' etc is perfect, if asked why then just something like 'Mum and Dad aren't happy living together anymore' is fine, not putting the blame on either parent (as difficult as that might be where cheating is involved). Try to keep routines the same in each home to avoid unsettling them, reassure if child starts asking e.g. 'does Daddy not love me anymore' that it isn't about them and ensure quality time with Dad.