Battlefield has a built in FPS reading output.
You can turn it on by first pressing the tilda key ' ~ ' (top left of most English keyboards - to the left of the '1') and then type "render.drawfps true"
You can then turn it off by changing the true to false
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)/
Run FRAPS before starting BF3
Load a section of a mission that is graphics intensive(plenty of smoke,destruction,enemies and explosions).
Press F11 or whatever the hotkey you have set for benchmark.
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.
Repeat this to get an average min,max and average.
Note: Make sure you have the MinMaxAvg dialog ticked under benchmark settings.For legit Benchmarking, this article and links provided within it may interest you.http://pcformat.techradar.com/2012/02/06/overclocking-everything/