Choose the date first
The Taj Mahal is closed to general visitors on Fridays. Check both sites for your date, and budget for queues and the approach from the car drop-off rather than counting only time at the monuments.
Make two stops the plan, not three
For an early Delhi departure and a Delhi return, prioritize the Taj Mahal and Fatehpur Sikri. If Agra Fort is more important to your group, swap it in rather than assuming that all three are comfortable.
Stay overnight if you want an easier day
An overnight is a better fit for travelers who need breaks or who want sunrise and several sites. On a same-day itinerary, do not promise yourself a fixed dinner, train or flight immediately after the return.