3 is fine - your daughter can become close to her brothers if you promote that relationship - plus if she has cousins - girl cousins can be almost as close as sisters.
If you would adopt - you might be taking a baby from a couple that don't have/can't have any children.
You're just having baby fever - honestly - it goes away. It's like a virus caught from holding new babies and smelling that baby lotion smell.
I know you're happy that you've been so blessed with the children you have- and you really have
It's fantastic that your wife is a full time mom - the best thing in the world to do.