I'm10 weeks pregnant I had a gush of fluid with light bleeding. Went to er. Doctor found heartbeat! He said everything looked good. I still have brown discharge for 3 days. Could I still miscarry?

It's possible. Many women experience bleeding in the first trimester of pregnancy. You did the right thing by seeing a doctor immediately. You experienced bleeding but no signs of miscarriage at the time. However, you are still at risk for experiencing a miscarriage with your history of bleeding. See your doctor immediately if bleeding or cramping worsens.
Yes. There's a 50/50 chance you could still miscarry. This early in pregnancy, there is nothing that could really be done. In the mean time, rest and take it easy, no sexual activity or other strenuous activity.
Yes, you could. A heartbeat at this time tells you that everything is fine-at the time the heartbeat is heard. A miscarriage can still happen, but is less likely, the farther along you are-esp if your doc does an ultrasound and it is normal. Lots of women who spot early in pregnancy go on to have normal deliveries, but keep an eye out for heavier bleeding or unusual pain-reasons to call your doctor.
Misscarraige. I know your happy to be pregnant, but 30% of pregnancies end in a misscarrage. Not all genes are successfull. Cross your fingers, have some "pevic rest", and let nature take it's course.