What you are seeking would not be a triad. It would be a vee. A triad is one of love and partnership between three people evenly. A vee is quite often a relationship of friendship (metamours) between two arms of the vee and the person that is partners with both is the hinge. If you don't love this woman then don't force it... just let it be and enjoy. I live with two male partners that are close friends and we have had threesomes together, but they are not in love and do not consider themselves partners.... partners in crime when they bug me however
but not lovers. There is not reason that they HAVE to be... it just happened like that.