Hi mate, welcome here...
You had a lot to digest i see, and even writing all these things down is a step in the right direction
Man this is quite a story...first things first, no matter how rude/awkward/downright di*k-ish it may sound, get, or at least ask, for a paternity test. This is one very intricate web of lies and deceit and you need to really know where you stand regarding this child before going off in any direction. If she gets mad, makes a scene or starts launching accusations, pursue it till you get your answer (a legal one i mean): sorry to burst her bubble, but it doesn't fit the timeline: if the 2 of you were together for 3 months, why would she ask the other guy for confirmation? Maybe some female members would like to fill this void, but most of the women i know would freak out if they missed their period right about when it was due time, let's add another whole month for the sake of the story....but hell, 3 months later? This is plain BS. There are a lot of red flags regarding this conception story and friend, i'd go straight for the paternity test at the sight of a slightly yellowish one, not to mention you have a box full of them.
The other part of your story....i know you love this girl, but step back a little and re-read all you wrote...do you really like what you see? is in your story even a glance that this girl wants to be with you? Bluntly said, you're a doormat, the safety net, the last option, call it however you want to call it, but you're no boyfriend, you're no husband and i'd bet some cash you ain't even a father. Let's leave the "father" aspect on a side until you get your answer and focus on the relationship aspect...this prompts the question "What relationship?" Hell man, each time she either meets a dude, either someone else bluntly walks all over you and sets her up with someone, she goes straight for it. Your "relationship" with her looks more like "the breaks she takes between other relationships while she is on the lookout" . Another problem is her availability...friend, to her an her cohorts, you are the last option, not even an option at all : she encourages her friends to always be on the lookout for potential partners for her, what does that tell you about your place in her eyes and the respect she has for you? And let's be blunt here, she ain't testing the waters, so far is there any "relationship" she had, that didn't included the sex?
That was the long version...the short version is:
- if you're the father - take care of the child BUT WALK AWAY from her
- if you're not the father - WALK AWAY and start running , there's no way to "fix" this, you can't make people love you and want to be with you and look again: nobody loves you here.