The first day of my last period was on june 7. I had aex with one guy on the 18 and another on the 28. Is it possible that baby was conceived on 28th?

Yes. In women with regular monthly cycles, ovulation occurs around day 14 (day one is the first day of the cycle). Conception can occur with intercourse a few days before or a few days after. Unfortunately, you can ovulate a little late and many women do. So it is going to be really hard to tell exactly which date you got pregnant.
Yes. It is possible that conception could've occurred with either sexual encounter. Assuming you have regular 28-day menstrual cycles, it's probably more likely that you conceived on 6/18, but it's not a certainty.