This is going to sound harsh, but only because the phrasing of your post suggests that you yourself are aware of what the 'right' response to this situation is. However, you're doing your best to convince us (read: yourself) that it's acceptable to go ahead with the threesome, because you're so excited by the idea of it happening.
If you have not been honest with your partner about the nature of this other girl's feelings, then you are effectively lying to her to satisfy your own obviously unconcealed desire for the three-way, which is extremely selfish. It also means you should not marry her because you are being dishonest with her. If you're lying before you're even married and potentially subjecting her to this, then wow, you're in for a very miserable time.
"also the other girl (Shouldve mentioned this) said that any feelings she has are under control and she would not let them become a problem."
I seriously doubt that. I'm sure she would like them to be under control and she tells herself that they are, but the simplest explanation is the most likely explanation: she will say whatever she has to if it will afford an opportunity to act on her desire for you. She can't be with you because you are with somebody else - this is, in her estimation, her only option.
Given the aforementioned, if you try to push for this to happen, you are betraying the trust, respect and friendship of both women and deserve neither of them, because your interest in boosting your own ego and having a good story to tell are taking precedent.
Furthermore, do not underestimate how explosively bad this could go. I'm not jealous of you because I've done this before, several times. At best it can be fantastic and at worst it will destroy your relationships and show you up for being a self-centred pig.
A threesome is not the be-all and end-all of sexual encounters. If you do end up doing it, then you're building it up far too much.
If you have not been honest with your partner about the nature of this other girl's feelings, then you are effectively lying to her to satisfy your own obviously unconcealed desire for the three-way, which is extremely selfish. It also means you should not marry her because you are being dishonest with her. If you're lying before you're even married and potentially subjecting her to this, then wow, you're in for a very miserable time.
"also the other girl (Shouldve mentioned this) said that any feelings she has are under control and she would not let them become a problem."
I seriously doubt that. I'm sure she would like them to be under control and she tells herself that they are, but the simplest explanation is the most likely explanation: she will say whatever she has to if it will afford an opportunity to act on her desire for you. She can't be with you because you are with somebody else - this is, in her estimation, her only option.
Given the aforementioned, if you try to push for this to happen, you are betraying the trust, respect and friendship of both women and deserve neither of them, because your interest in boosting your own ego and having a good story to tell are taking precedent.
Furthermore, do not underestimate how explosively bad this could go. I'm not jealous of you because I've done this before, several times. At best it can be fantastic and at worst it will destroy your relationships and show you up for being a self-centred pig.
A threesome is not the be-all and end-all of sexual encounters. If you do end up doing it, then you're building it up far too much.