Google is your friend and can answer this question quite well. Almost all bulletins stack, but they're multiplicative. For example, when dealing with USF AT gun or Soviet Guards:
pathetic_penetration_value * 1.04 * 1.04 = infitessimally_less_pathetic_penetration_value
so what now? do 2x +3%acc bulletins give 1,06%acc or (1,03 + 0,03*1,03)acc ?