Can't give you exact req. for scene to trigger I guess everyone need to have high lust and GF needs to like both Tina and Anita and most likely have some cum addiction + RNG aspect for GF and Anita to come.
To trigger the scene you need to go into cafe when Tina is there talk to Tina and give her blowjob behind the counter can take few tries cause often she talk about work etc. leave or offer you coffee.
Also, don't worry you get more action in that scene even if I personally kinda enjoyed 3 way with Female MC.