Welcome to our forum.
As far as terminology is concerned, it really depends on whether your relationship with Ashley is platonic or romantic. If platonic, then you are in a Vee with Miguel as the hinge. If romantic, then you are in a triad (a.k.a. delta or triangle), with all three people romantically connected.
I think all relationships are different, and you have to find out what works for you. Having talks, say, once a week or once a month, between the three of you, will probably help, and writing up lists of wants/needs to share with each other may help too. During perhaps the first of these talks, tell Miguel and Ashley where you'd like to go from here, goals you'd like to suggest, and find out what they want and envision for the future.
As for reassuring Ashley, one of the best things you can do, probably, is to set aside at least a few hours a week to spend just with Ashley. "Girls' time" or "girls' night out." Developing a closer relationship with her will help her feel like she can trust you.
I am happy to answer any other questions you may have; hope this will do for starters.
Love means never having to say, "Put down that meat cleaver!"