I will assume that you have FRAPS(if not you can get the free version from FRAPS.com;the main difference between the full and the demo AFAIK is that in the demo you can record videos of upto 30 seconds at a time) . <br>
1)Run FRAPS before starting BF3 .<br>
2)Load a section of a mission that is graphics intensive(plenty of smoke,destruction,enemies and explosions). <br>
3)Press F11 or whatever the hotkey you have set for benchmark . <br>
4)Play your normal game while FRAPS benchmarks for <insert time here> .Press F11 again to stop benchmark . The min,max and average FPS will be stored in a Spreadsheet file.<br>
5)Repeat this to get an average min,max and average .<br>
Note:Make sure you have the MinMaxAvg dialog ticked under benchmark settings .
![enter image description here][1]


  [1]: https://i.sstatic.net/QcBsb.jpg