Hi i had 3 miscarriages and every time the g sac can be seen but no fetus or heartbeat what could be the reason?

Genetics: Unless there is a problem with the uterus itself, almost always the pregnancy is genetically abnormal. That has been known for a very long time. Some women even at a young age can have that problem. It takes advanced fertility therapy to diagnosis and fix that problem. Although there are some other possibilities likely blood clotting disorders for instance, genetics is the most common explanation.