About to have a threesome, got a few questions...

I'm in an awesome LTR with this girl (31 year old here). She complements my life very well right now (super submissive, not into social media, cooks, cleans, low n-count etc). I've been molding her into the woman that I want (she's adapted to my lifestyle, which is nice). Anyways, I told her a couple nights ago that I wanted to have a 3-way with another girl involved, and I was surprised when she said she would be down. I casually asked her why she would agree to it, given that she loves me, and her response was that she didn't want to hold me back from my desires and fantasies. She also said that the idea would make her want me even more (seeing me with another girl), and have her see from the perspective other women look at me.

Anyways, I've never had a 3-way with two girls before, but I'm down as fuck. Is there any short-term or long-term drawbacks from opening this up? I am happy with the relationship currently, but will this fuck up the relationship in any way (mind you I've accepted that at any point in my relationship, if it doesn't go my way, I'm making her walk the plank)? Also, I got a few younger girls that are clearly interested that I've been talking with on the side, is it a bad choice to go with them? Just not really sure what I should expect besides an interesting night(s) with two women.