The Picts didn't exist yet, and the Senone were under the control of the Aedui, so wouldn't be a seperate faction. Picts came about as a response to Roman expansion in Britain; fleeing Britons allied with Caledonian tribes in the north and formed a confederacy. That's way out of our period. Also, as an aside, Senones did not single-handedly sack Rome, they had other allies in Cisalpine Gaul who came with them. Senones were merely the leaders of the sack, under their then current king Brennos.