Hello Tyy
This isn't really a good situation for any of you.
All three of you are in a 'relationship' based on lack of honesty and trust.
Perhaps, if you had asked her out before, then this other boy would not have been in the picture ~ but he is, now, and this isn't fair on him.
There is a chance that he will find out what is going on, which won't be good, either.
Do you want to carry on like this?
I'm guessing that you would rather share her with him than give her up altogether?
Have you talked to her about it?
That could be the best thing ~ as long as you accept that the result may not be the one you are hoping for.
Good luck!