If you want to display your True FPS (frames per second) in Minecraft, then use a free application such as FRAPS or MSI Afterburner.
The app which I use is FRAPS. You can get the Free version from here - Here
Simply install it, open it, then keep it opened in the background and your FPS should show in game. Hit F12 on keyboard if it doesnt show or you want to move the overlay in different locations.
You can also get MSI Afterburner to get more detailed info from Here
And remember - NEVER use F3 to view your FPS in game. It displays the wrong amount at a lower rate. Always use FRAPS or another application to view it.