How come the Rangers stopped using their Power Weapons during MMPR Season 3?
Most likely because the Power Weapons were no longer sold as toys (see Season 2’s replacement for them: The Power Cannon, which was in turn ditched in Season 3). Blade Blasters were technically part of their costumes, so, much like how Saba would rarely be talked to or mentioned during Season 3, anything that wasn’t new or toy-sellable got the shaft.