Dear Julez,
I feel for you dude. I really do. You are probably not in the right frame of mind to hear this right now, but I think I need to be blunt.
I've been through this before, although it sounds counter-intuitive, there is only one way to save your marriage. You have to be willing to walk away before she will come to her senses. She is clearly deep in the fog and in la-la land with OM. You have kids so this complicates matters, but in fact may help snap her out of the fog more quickly. Here is what I suggest. First of all KEEP CALM AND DON'T LET HER KNOW YOU ARE DOING ANY OF THIS. ACT LIKE NOTHING HAPPENED.
1) Is OM married? If so get OMW contact details. If not then find out who his close friends and family are. You will need this later during D-day.
2) Who is your wife close to? Any close girlfriends? Sister? Pastor?
3) Gather up your evidence about the affair. Emails, facebook screenshots. Do you have access to her cell phone? If so look on it for text messages. There are software programs out there to help you retrieve deleted text messages and voice mails. Once you have her cell phone install some spyware on it. Turn the GPS tracking feature on her phone on so you can track where she is going. You will need this because she will surely continue to contact the OM once you confront.
4) If you don't have access to her phone then buy a couple of VARs and leave them around the house and in her car. Sony ICDPX312 is the best one. Buy some heavy duty velcro and secure
5) If you don't have access to either her phone or cell phone then buy a keylogger and install onto the computer she uses. There are software keyloggers and hardware based ones. If she is tech saavy then go with the hardware type. If not and if you have admin privileges on the computer install a software based logger. This will help you in the future keep track of whether or not she is continuing the A with OM.
6) Go see a lawyer. Get D papers drafted. Does Canada have infidelity as a grounds for D? You can file the D papers but it takes several months to finalize, so you can always withdraw later if she comes to her senses. But you NEED TO BE PREPARED TO FILE.
7) Once you have enough evidence, expose the affair to her close friends, your family, your kids, her family. If you expose via FB be careful not to send more than 1 message each minute else it will mark yours as spam. Tell them you want to save the marriage and need their help. Ask for their support. She will FLIP out that you told everyone, but this is critical. Be prepared for a big war. But be firm, that you want to save the marriage and sought help from everyone you thought would care. Affairs like the dark. Shine the light and the A will wither away to nothing.
8) The most critical person to expose to will be OMW. Once OMW knows, OM will likely throw your WW under the bus. Be sure to expose to her in a way that it can't be intercepted by OM. Direct face-to-face contact is best. Show her enough evidence and ask for her help in getting both waywards back into marriage.
9) Confront. Corner your wife in a place where she can't freak out and explain calmly that you are giving her an ultimatam: him or you. She cannot continue to see OM and have her family (cake) too. Explain that you will not be treated like a doormat and won't stand for this. She can continue to see OM if she wants, but you will be filing for D. IF SHE FALTERS EVEN A TINY BIT AND SHE DOESN'T IMMEDIATELY CRY AND BEG FOR FORGIVENESS THEN IMMEDIATELY FILE THE PAPERS. this will be a critical moment. If she starts to blame you for the problems in the marriage and wants to argue, just walk away and say that you don't want to talk about this right now and all you are interested in is saving the marriage. Then File the papers. This is the only thing that will snap her out.
10) If she begs and pleads on confrontation day, then immediately tell her that she must agree to the following: 100% no contact with OM for the rest of her life, write a NC letter, change her cell phone # and block OM, change her email address and block OM, STD testing, IC and MC for you both. If you really want to throw her for a loop then ask her for paternity testing for your 3 kids (this will let her know how deeply you have been affected and mistrust her now)
Once you do this, come back here and post and we will tell you what to do next. I'm so sorry you are here. But we will take care of you, man.