Well, of course you're nervous. I've been dating since 2009 and I still get butterflies before a first date with someone. Just ride it out, and keep an open mind. For me, nerves calm once the conversation starts.
If she's never dated a couple before, and you two have never dated as a unit, are you aware of the common pitfalls of "unicorn hunting" and the rights of secondaries? Triads are notoriously hard to manage. THAT makes me nervous for you.
It's much different to have a long time gf to be "shared" by an established couple, than a one night stand 3some. Almost always, feelings will be unequal between one dyad or another.
Are you prepared for her to just prefer you, or just prefer your husband, or for one of you to like her but the other not, or for one of you to like her, but she doesnt like you, and you feel rejected as your partner gets all the lovey feelings returned while you don't?
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
"Master," (mono, 36), miss pixi's Dom for 3 years