ABUJA, Dec 2 (Reuters) - Nigeria"s telecoms regulator has picked MTN Nigeria (MTNN.LG), Airtel Africa (AAF.L) and Mafab Communications to bid for 5G licences at an auction set for Dec. 13, it said on Thursday. The Nigeria Communications Commission had said in July that it expected to start auctioning spectrum for the next-generation 5G network in the fourth quarter of this year.