my girlfriend is obsessed with me having other girls

Okay, so it stopped being funny last weekend. Should I be scared? or is it some type of new perversion I don't know about?

I've been with my girlfriend for over two years, and not once has she been jealous when other girls call or hangout with me. 

Later on she started to encourage me to have sidechics,  at first I thought she was testing me, I found out she was serious, she still shows affection for me as always even with the sidechics (I cupped me a few).

Last weekend we were talking casually when she proposed a threesome with one of my sidechics, at this point, people of the world, I'm either scared or confused (can't really tell which or both).

Can't give much more details here, but dear #steemsages, I need some advice or experience