Because the third time is too soon to ask for a threesome. First time, "hurray for sex" second time, "yes, it wasn't a pity fuck" third time, "OK, what's next?" Anal is "next". (A threesome is a different next, but on the third "date" is too soon)
I think every every person has a different timeline for when they think sexual events should take place. Even as far as how long to wait before you reach out to someone after getting there number! I think if I person really enjoys anal they should be albe to ask for it when they feel like it...they might not get it but you can always ask
My wife and I were married a couple of years before we did it. She had said had tried it with her ex, but he was too rushed and rough. With a lot of patience and lube we did it, and she loved it. She is the only woman I've had anal with that orgasms just from having me in her ass, with no additional front door stimulation. She loved feeling me cum "back there"!
My first wife was the same way. Once I was inside her ass she went wild riding me so she could hit the right spot to trigger unbelievable orgasms. She wanted me in her ass all the time after that. I'm bigger than most at 8" and she'd bottom out to my balls bouncing hard on it and screaming like a hyena. She was a great fuck for 28 years.