According to me, you should first try and talk with your friend regarding this. You should tell your best friend about your feelings for his wife. Maybe he can allow you to talk with his wife which can clear your way further.
It might not be love. You may be attracted by her or having a lust on...
I may advice you to balance your life with each partner in an equal way. Maybe this can help you to give each partner same amount of time. If this doesn't work then cut down on your partners from 3 to 2 or maybe only 1.