I'm afraid I have to echo what others have said.
First you need to take a cold hard look at yourself and what you want (and maybe a side-order of why you want it). Be very very honest with yourself (if you can't be honest with yourself, then you can't really be honest with anyone else).
How important is the truth to you? How important is it that you have a truly open and honest relationship with the both of them? If the truth isn't as important and you would rather go through life essentially doing what it takes to hold on to both of them, then that is your decision, but if you decide that then nobody on this forum will be helping you, because that breaks on of the necessary definitions of poly.
Once you have decided this, and if you have decided that you really need this to be open and honest then you really need to start talking with them openly and honestly about how you feel. I won't pretend with you that this will go well - they will be upset and angry and feel cheated on (because, let's be honest, they were). The question is whether they want to continue with this. Yes you may lose both of them once they both realise that you have lied to them both. That is the risk of cheating first, and then wanting to be honest. You will almost definitely have to go without sex for a while with either of them.
When you have got to that point, you (all involved) then need to start thinking about all the stuff that poly partnerships need - and there is ample reading on this subject out there. Is this something that you all really want in your lives? Which of the typical poly configurations will work for the three of you (note that I didn't say "which one do YOU want").
This is not going to be easy, and I'm not going to pretend that it will be, but I wish you luck.