Why didn't she use all 3 dragons?

She didn't need all 3... she could wipe out the whole army without risking all 3 dragons.

I was wondering the same thing. Then I thought it could be one of two things or both.

1.) She left two behind to defend Dragonstone so they aren't ambushed the way her sea fleet and Highgarden was.
2.) To minimize casualties so she can turn the surviving enemy soldiers to her side to strengthen her army. (As seen in the preview for episode 5)

She wouldn't be able to control them all at the same time, at least not in a specific way. When riding Drogon, she can direct him to specific targets and shit. If she brought the other 2, they would have just killed everybody.

I'm also wondering how well the dragons would be able to distinguish between the enemy Lannister army, and the Dothraki. They could end up killing tons of Dothraki too.