My first response is purely an assumption based on these two things:
"I made it explicitly clear that my involvement was optional"
"when she has a concern she only talks about it to our boyfriend"
It does sound very much, from the outside, that your Unicorn might well be thinking of your boyfriend as her main person, main relationship, with you as an extra person, rather than thinking of the two of you as her equal partners/lovers.
To have you 'involved' in this way means that she's saying "sure, have sex with us, that would be fun" - but that her relationshippy side is pointed towards him.
As everyone else had said, some clear communication and roles are needed.
Decide for yourself, before going into the discussion:
- Are you happy for him to date her alone?
- Are you happy to be an occasional 'extra' / purely sexual extra?
- If you are having threesomes, but she is more focused on him relationship-wise, are you happy to let them do their communication thing?
- If you are an 'extra/optional', what do you *need* her to talk to you about, specifically?
If you get those things straight in your head, you should be able to express yourself with clarity. I have an inkling that she's asking boyfriend "is this ok?" because their relationship is developing differently (i.e. more intensely) than the threeway dynamic.
I do think that it is rare for three people to find each other equally as attractive or appealing - in my experience, two people often end up developing something deeper. i.e. for you guys, threeway sex, relationship between you and boyfriend, relationship between her and boyfriend, you and her in a peripheral 'relationship'/friendship.
If you do agree to only be a sex partner, or to be happy with them getting more involved as a duo without you, then some of the communication stuff might be flexible. Of course she should come to you directly - that would be ideal. But if she's feeling closer to him, she might be chickening out - so, open the lines of communication with her and find out what it is that she really wants and how she really sees the two of you.
A final point to consider...
My girlfriend's husband was hopeful that I would be their Unicorn, for a very long time. I am not attracted to him, but I was open to (and did engage in) mild threeway play here and there. I was more open during the first 6 months. I did this because I was neutral - *not* because I was interested in him. I just thought it would be good for my girlfriend and wanted to give both of them that. It might be worth finding out for sure whether your Unicorn is neutral and doing it for the thrill of a threesome / for your boyfriend, or whether she is actually sexually attracted to you. The best way of determining that is by asking whether, hypothetically, she'd be interested in having sex with *just* you. If she wouldn't be interested, she's not really interested in you. If she would, she obviously is interested - but either there is a communication breakdown, or she's *more* connected to your boyfriend.