what if the guy your crazy about digs you back but an outside person will end up getting hurt? is it worth it to go with the first one? or be happy with a mediocre situation?
You can not always worry about everyones feelings, think about yourself, do what your heart tells you to do!
Don't cheat or be a party to cheating. If you are the one with the significant other, then you need to break up with them before you pursue a relationship this other person. If the other person is the one in the relationship, you need to make it clear to them that you will not start anything until they have broken up. Anything else would be unfair to the third person and very immature on your part.
At 17, maybe you are not as ready for intrique as you might wish. With a few years of experience, you would at least consider stepping back and trying an objective view of all parties. Since you asked a question, I'll answer. Step back, look at the cost. Are you really responsible for hurting another human? Try walking away from the whole deal. Get another 'friend.' You may be not ready for all the hurt. Sorry to sound like an old fart, but hurt is hurt, and there is already too much in the world. Take life slowly to enjoy it!