1) forgive her and take an undertaking from her that she wont indulge in any extra marital affair in future
2) for the sake of your 3 year old son try to save your marriage
3) visit a family counsellor to resolve your differences
4) you can if you so desire file for divorce on grounds of adultery and mental cruelty .
5) you can also file case of adultery against her lover under section 497 of IPC for having sex with your wife without your consent even though he was full aware that she was a married lady
6) service provider dont maintain records of chat details