I'd say ur ex. Those dates were when u were most fertile. Good luck
I'd say almost 100% that it is your ex. It is very rare to ovulate right after finishing your period or before 10 days have past since cycle day one. The match up of dates with your ex is right on the money for a normal cycle time for ovulation and conceiving a child. good luck
Unless you have very short cycles and so ovulate a few days after you finish your period it;s unlikely you would have gotten pregnant from having intercourse with a friend on October 8th - the day after you finished your period. Most like the father is the one you had intercourse with October 15th and 16th - because if, like most women, you ovulate mid cycle - then you would have been due to ovulate around the time you had sex here.
Obviously like you said, DNA will be the only way to know for 100% when who the father is, but going by when you had sex I would say it's most likely to be your ex.
You can't accurately track down conception. So you can't know till the baby is born and you do a DNA test. Since how far along you are you should probably tell the guys you slept with ahead of time that they might be expecting a baby. Even if you don't think they'll tale it well it will be so much harder when that baby is born to tell them. And also one of those boys are the father the baby is half of one of them and they deserve to know.
Good luck