Further to SFC Lee's reply, 3rd person usage is intended to be used to increase awareness however; information gathered that would not be seen if playing in first person should not be acted upon. For example, you cannot typically see over top of an H-Barrier in first person; if you're in 3rd person you can.
Typically this is most relevant in a MOUT situation when clearing a town, third person is not to be used to see over walls.
I personally utilize first person as much as possible as I prefer the immersion provided by that perspective.