If you started your period on 3rd March then your LMP was on 3rd March. The date your period finished is, to a large extent, irrelevant. Some periods last only 3 days; others (like yours) maybe 6 days. Your menstrual cycle is considered to start on the day your period starts. So 3rd March is Cycle Day 1.
As Medic-Dan said in reply to your other question, the fact that you had a normal period in March indicates you were not pregnant then. You are very unlikely to have got pregnant from sex on the first day of your period.
You haven't said how long your cycle is, but a typical cycle is 28 days, so that would mean your next period was due 31st March. That would indicate that you may have ovulated 17th March (CD15). That being so, you would have been considered fertile on 15th & 16th March. The sperm deposited in your vagina would have reached your fallopian tubes and waited for the release of an egg on 17th March, when it would have fertilised that released egg. Implantation could then have taken place about 26th March. That would have been when you actually became pregnant.
A due date is often calculated from the start of your LMP, which would be 40 weeks from 3rd March, taking you to 8th December. However if you (wrongly) gave your LMP date as 8th March, then the due date would have been (wrongly) calculated as 13th December. Of course, the actual birth could happen any time now.
What you haven't said is what prompted you to take a pregnancy test on 29th March when, assuming a 28-day cycle, you had not missed a period.
I hope this helps