First, realize that you are entering a transition phase that is supposed to feel strange, flat and off-balance. You've lost your bearings and buffers, formerly known as kids. You were busy doing the good work of lighting fires under the kids, but now you have to readjust and light a fire under your partnership. The good news is that once you make this crossing, you may be entering the happiest time of all for any marriage. But it is a challenge.
Divorce goes up as couples reach the empty nest stage so there is work to do.
It's time to put your relationship on the front burner. So, first, count your blessings. You've survived the hard part and launched three kids. Make lists of all the things you'd like to do. The world is so full of things and they are best enjoyed with a partner.