It depends entirely on the taste of the company's management team and/or Chief Pilot. Every company I've worked for except one, F/Os wore three bars. Two of those companies used the grade SFO and there was no distinction in rank; one company used two bars for FOs who had not yet finished their line training. First company I worked for used two for FO and three for SFO but upon the arrival of a new boss, all FOs got a third bar. I understand that when he went elsewhere, his replacement reinstated two-bar FOs.