I am 19 weeks and 4 days pregnant my estimated due date is june 24th,2023. my last period was september 17th-21st right after i was having sex with my current partner the 24th-1st. i slept with my ex the 3rd. which is most likely the dad?

Too close to tell. Once a 2nd potential father enters the picture it becomes a legal issue where the true father gains certain rights/responsibilities. You can receive a paternity study as early as 8-10 weeks on maternal cell free fluid (NIPS) that can provide evidence that will be accepted in court. Test or not, over time the answer may or may not become quite evident.