Krispy, I'm so sorry to read this. Everyone has their own response. My history is similar. My WS had a twelve year 'love' affair that I found out about 23 months ago. He was also in another relationship that had endured for 18 months concurrently. What I additionally found out six weeks later was that there was a previous affair that lasted for ten years and various others. It sounds hideous, I know.
What I can say is though is that my WS has done everything that I have asked of him. He dropped the one he 'loved' (it wasn't real love - how could it be?); and has been in IC and MC and I am now the most important person in his life. He was like a deer in the headlights for the first two months or so and it took a good year for the 'fog' to lift but he was determined to work on the marriage and himself after I found out.
Three weeks is such a short time. I am almost two years out. It's been hell but it's getting better. A marriage can be worked on if two people want to make it so. You need real glue underneath and a real friendship too.
Please take your time to consider your options but don't simply take your WS's words as gospel truth. Use your forensics and check everything for evidence of anything else and watch him! Watch his actions, his body language, his responses to your questions and read as much as you can. 'After the Affair' is a must and is certainly a book that should be read together.
MC is for the both of you once all the truth is out. No point if he's still hiding or foggy.
Please read as much as you can here but you can't take ALL the advice that is offered.
Please keep posting too. It's a great site and a wonderful help - especially in the beginning.