Finances-number 3 hasn't made a huge impact 2 years on. Suspect will feel it more when starts school/clubs. Things like holidays, Christmas and days out are obviously more expensive. We budget though. Things to consider are space in the house and is your car big enough.
Body-I gained the most weight in the third pregnancy due to having to be on bed rest for a few weeks. I lost the weight pretty quickly however.
Relationships-no impact negatively. Kids adore the youngest DC and me and dh although busy, very happy. So much so, we are having number four.