If the parents are divorced, the father is remarried and the mother is in a relationship with someone for 3 years - where do they all sit

I am assuming you mean the sitting arrangement for a wedding in a church, right ?

If all get along well together, and the people you mention are on the bride's side, then on the bride's side of the church as it should be. If the other way around, then on the groom's side.

Surely for a happy occasion such as this, everyone can get along to make it a proper wedding with no misgivings.

Then on the other hand, if it is at the reception for the main table, then I would wing it and just seat the wedding party at that table and let everyone else, parents and all have their own table, or wherever they want to sit. As far as I can see nowadays, everyone does their own thing instead of following tradition. There are so many parents broken up these days, there is no other option then to play it by ear.

Ms tickle