Getting back to the main issue.
You had sex on the following schedule. It was all unprotected.
Guy 1 January 2
Guy 2 January 3
Guy 3 January 11
If you go with the 8 weeks 2 days / 8 weeks 4 days counts from the first ultrasounds (which is the best you have to go by, later ultrasounds do not matter because babies diverge in their growth rates from the average after a while), that gives a suggested conception date of January 13 with a margin for error of +/- 3 days either direction due to the ultrasound being in your eighth week. Sperm can live 4-6 days in your body. This collection of data gives the possibility that the last guy, the one on January 11, is the dad, rules out the first guy, and pretty much does not suggest the second guy either.
But if you use the due date given to you by the second doctor (which is a puzzle because if the ultrasound really said 8 weeks 4 days, that's not the correct due date), but if you do use it, then the second guy is a little more in the realm of possibility.
You didn't say whether the baby was a boy or a girl. If the baby is a boy, it would be moderately indicative of the second guy not being the dad.
So your first ultrasound was on the 26th and said 8 weeks 2 days, and your second ultrasound was on the 8th and said 8 weeks 4 days? (If so, they should both have said the same due date. I'm surprised they didn't.)
If your original doctor was correct in saying your due date was October 6, the conception calculator gives January 13 for conception.
How fast after that was the other doctor appointment (where they told you the due date was October 1)? If it was, for example, the next day after the first appointment and what she said was based on an ultrasound, then you have a difficult difference of opinion to sort out. But if the second appointment was weeks later than the first one, and/or if you didn't have an ultrasound at the second one, it is less surprising that the estimated conception date would be different. Some babies grow faster than the average and some grow slower. Because of this divergence, woman are often told a different due date at a later point in pregnancy. But a fast-growing baby does not change when conception was, and the earliest ultrasounds are the most accurate for trying to guess when conception was.
You also said you had a 12-week scan (that said such and such) "so I guess they are sticking with October 1." But you don't mention when the 12-week scan was (on the calendar, not in weeks) so I can't offer an opinion about whether they are sticking to October 1 or not.