I am planning to text a female friend in front of my ex-gf 3 days after we broke up
Why would you want to text her at that moment?
I want to make her jealous.
Hurting your ex's feelings makes you feel good temporarily, but it is not the answer. What you need to do is move on with your life.
It's a good thing I told her what I was going to do before I did it.
Your ex is your past and since you've found someone else to care about, should be forgotten. You'll feel a weight of burden lifted off your shoulders when you stop stressing about what your ex thinks.