dude, seriously, you are selfish. No matter what, NO MATTER WHAT, a friend's girlfriend is off limits.

"I would hate to lose my friend but at the moment I'm at a point in life where I'm really desperate to get with another girl simply because of a breakup that happened 3 months ago"

You said it yourself, you're desperate. And basically you would feel kinda bad for your friend, but screw him, right? You want his girl. Geez would it kill you to be single for a while and not leech off other people's relationships? Don't interfere with other people's lives, especially if you're the only one wanting to gain something. Maybe your ex was onto something...