SMU is the safest way to set it.
# smu_cli set tx-max-bg 6
# smu_cli get tx-max-bg
Reg Domain: WW
BG Antenna Adj: 3 dB
A Antenna Adj: 0 dB
Remember the value is in ½ db increments so you need to use 6 to get 3 dBm.
Also fw_setenv is able to set it as well.
rfPowerAtten2=6
# fw_printenv
...
usbnet_devaddr=de:ad:be:ef:00:01
md5_at91bs=75a5a900d8df5657ebc89796756eed70
md5_u-boot=cc8187cf4471832e51580a284df37f86
md5_kernel-a=300d5b9d93abaec368fb700933a95bdb
md5_rootfs-a=62e98ead848d552ab6199b6ed543dd4d
bootargs=console=ttyS0,115200 loglevel=4 rw noinitrd mem=64M rootfstype=ubifs root=ubi0:rootfs ubi.mtd=6
bootcmd=nand read 0x22000000 0x000e0000 0x00500000; bootm
rfPowerAtten2=6