I have had 3 normal heavy periods (normal for me) and 3 negative urine tests, 2 at home (first response) and 1 at doctors. could i still be pregnant?

Unlikely: It is unlikely that you are pregnant - why do you think you might be with every test negative?

Not likely: Serial pregnancy tests are the only way know for sure.

Each time you have : Your period, it means that you are not pregnant at that time. If you have been sexually active, first response may detect pregnancy up to 6 days prior to your period and a standard pregnancy test can be done when a period is missed.