It could very well be that a triad with her is not going to work. That is totally fine. They quite often don't when approached by way or a couple wanting a unicorn to fulfill their needs. I don't know how you all started off but they seem to work better when there is a friendship before and unexpectedly love between three emerges.
There is not shame in you not loving her the same way as him and him not loving her the same way as you. Or even him not loving you the same way as him... love is not balanced and relationships are not balanced...what is, or should be balanced is time and energy and commitment to doing what you say you will do (integrity). The rest is kind of an interesting emergence of what fate has in store for you all.
It sounds like in your case your NRE between three of you is ending and a new phase is starting... so what will you do next while they start a relationship of two? What kind of goals do you want in terms of your relationship with him? What will you request of them to ensure that your needs are being met while they establish their relationship? What moves will you make to develop your friendship with her? What are your personal goals...
My point is that you can't fit a square peg into a round hole and if they are getting along dandy and you aren't fitting the equation any more, then maybe a shift needs to happen...
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