Stop using chemicals, including chemical fertilizer, while we sort this out. You left out the most important information.

Where do you live?

What kind of grass do you have?

What is your watering schedule? How often and for how long do you water?

How high/low do you mow?

Do you have a weed problem? What kind of weeds?

I agree with dchall here. Why are you putting down repeat applications of herbicide when there are no visible weeds in the lawn? Very likely you have damaged the grass putting down 3 applications of broadleaf weed killer in short succession. Read the labels. You have probably already exceeded maximum allowed rate for a full season. Lawns primarily need fertilizer and water, with some herbicides twice a year to kill weeds. Make sure to allow at least 4 weeks between applications of fertilizer and do not exceed recommended rates.