Is it wrong?

Me and my SO have been together 2 years in November, here recently I got in touch with an old friend of mine we will call her 'Amanda' (not her real name) well the discussion of a 3 sum has came up before my SO and me several times, I told him I would think about it but really it would just depend on the girl. Well after I thought about it for a while it started having me thinking whether or not I would be able to sit back and watch my man have sex with another girl etc, well since I've started hanging around with 'Amanda' here lately my thoughts have started to wonder and I find myself getting butterfly's around her. Every chance I get I touch her (not sexually) just like pull her shirt down if it rides up (which I love seeing) or just simply brushing up against her. The other day we hung out and she was going somewhere she didn't want someone to find out about well I can went with her and she made me promise I wouldn't tell anybody , I told her I promise as long as I got to touch her boob, well she let me and I didn't say anything . ever since then I have constantly wanted to do more with her, regardless if she would return. The favor or not. When I hear that song 'girl crush' I think of her. I wouldn't say I'm falling for her, but the connection I have with her is beyond amazing.

My thing is, I want to try to take it to the next step, I want to just fool around and touch, I would love to eat her out but I just don't know how she feels, I have brought the 3sum idea up to her, and she said she would but she is in a relationship, so I'm wondering if I should try or how I should go about telling her that I would like to pleasure her, I don't even want it in return I just want to taste her body, and I would love to just eat her out. Please any advice.