I have 3 tomato plants

Question

The plants are healthy and the blooms open but then before tomatoes form, the blooms fall off.

Answer 1

A good place to start to analyze your problem is this basic growing guide: https://www.gardeningknowhow.com/edible/vegetables/tomato/growing-tomatoes-guide.htm
This article also has some information: http://www.njfarmfresh.rutgers.edu/documents/BlossomDropinTomatoes.pdf