gf wants 3some with another girl

This is sloot territory which I'm familiar with. However my previous relationship...well let's just say ffm 3way wasn't in the cards haha...so my current is interested and has broached the subject. I joke back and say things like i have been on the hunt for fresh meat to bring home to her. She gets fake mad about me saying I'm speaking to other females but I know she is open to sex with a girl with me.

Been seeing gf for about 4 months. It's a committed relationship. She does everything for me nd is very submissive. Also she is 21. I haven't dated a girl older then 21 in a few years. Anyway I feel guilty talking to other women but knowing I have a free pass so to speak helps me out...so any advice

I can only speak from experience and I've had two threesomes in my life. While I enjoyed them at the time, they turned out to be huge red flags.

The first one was with a girlfriend I had been in a relationship with for about five years at the time. The other woman was a friend of hers. Shortly after that my then girlfriend suggested that we have sex with other people. I agreed. Here's the kicker: I suggested to her that we tell the people we sleep with that we as a couple are ok with it. She was vehemently against that. Lol. Needless to say that relationship died a few months later.

Second threesome was with a woman a few years ago I met on my first and only stint on an online dating site.
After a few dates I picked up some red flags and began to withdraw. She called me one night out the blue and told me a friend from overseas was visiting and they wanted to share me.
It was a great night. There were promises of doing this kind of thing regularly. I ended up walking away. Ran into her a few months later and we had lunch together and she told me she had bipolar disorder. It clicked and I realized the threesome probably happened during a manic phase of hers.
While I know people with bipolar disorder who have it under control and are high functioning..based on my past experiences, it's not the kind of person I want for a long term partner.

Saying all this to say, be careful with the threesome suggestions. They're not always as sincere as they may seem.

it's pretty much a red flag, plus the "she's a submissive 21 year old"

End well this shall not.

As above suggested, do teh 3some before you get devalued

Always stick it in your girl first before you do the other one. Doing it the other way causes problems.

If your gf gets up to go to the bathroom, don't be making out with the other chick when she gets back. Party foul.

Threesomes are great the first time ; by the sixth or seven time MEH. Too much work