What APTx Power Level Assignment would be required to increase power by 3 dBm on 2.4GHz radio?

Here is the power conversion table to go from mW to 3bm:

So, with a power level setting of 5, the output power is is 3mW, or 5 dbm. A power level of 4 would increase it to 6mW, or 8 dbm.