ps -- Be sure to get tested for STDs also.
It might not ever be possible to know by the dates, but your best bet is to go in for an ultrasound early in the week of April 7, and get an ultrasound. Go to a good ob-gyn not the ER, and have the ultrasound tech tell you the estimated due date. From that date, and that early in the pregnancy (either late 6th week or early 7th week), with a good ultrasound tech doing the assessment, you will get the most correct date possible that can be done by ultrasound. Then count back 266 days from the estimated due date, and that will be your estimated date of conception.
Unfortunately, you're in a position where the sperm from guy 1 was still alive in your system when you had sex with guy 2, and certainly the sperm from guy 2 was in your system when you had sex with guy 3. If counting back from your EDD gives you March 3, you can pretty much hope it's guy 1, but if it gives you March 8 or 9, while you might be able to bring it down to two guys you won't be able to bring it down to one.
If you have plenty of money to throw at this problem, you could get a prenatal DNA test from one of the two reliable labs out there who do this kind of work. If not, you are probably going to have to talk to all three guys and say you're sorry but a DNA test is in all their future once the baby is born. I'd have this talk after having the ultrasound, though.
Don't have the ultrasound too early because embryos aren't even visible in the 4th week and are iffy in the 5th week, and don't have it after your 7th week because there gets to be a larger margin for error as time goes by.
Good luck, write back when you get the ultrasound.