Things you must know :
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1)I used to hang out with 3 girls(girl A , girl B , and girl C) and one friend of mine. ( 3 girls , 2 boys ).
2)My friend started a relationship girl C.
3)I was being friendly with all 3 of the girls. Friendly enough to be considered charming , funny . Slowly i became good friends with them.
4)My friend broke out with girl C. Meanwhile a ''romance'' was starting between me and Girl B.
5)We stopped hanging out 'cause it was too difficult(girl C and friend didnt talk and stuff).
6)They continued talking to me , in school and stuff. They invited me to talk with them in breaks e.t.c.
7)I developed a crush in girl B , slowly. She liked me also but...
8)Summer came , and we cut all connections.
9)Now they are talking to me again. The ''crush'' i had is in the past - for sure -
10)Surprisingly girls A,B do not talk to me as they used to.. girl C though is talking to me. She even wrote me ''love letters''. Later she said she did it for fun e.t.c.
11)She comes to me every break and talks to me.
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Here comes the question :
What do i do? Should i make out with her? Or is she ''forbidden?'' Given the fact that my friend cut their relationship , she may stil have feelings for him.
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My deductions are :
1) She likes me - everythings ok -
2) She likes my friend - everythings ok - she will prefer me soon -
3) She is a good friend - for the time being -

What do you think? I kinda think 1,3 are more possible.

....

If they've cut ties, I don't see any reason for you to hold back. Especially considering it sounds like you're in high school. Go for it.