The TSB's investigation found the plane likely stalled as it was preparing to land in October 2013, before plummeting into some trees on West Cracroft Island, part of a series of islands east of Alert Bay.
Pilot Kevin Williams, who started working for Air Cab about 10 days before the crash, died in impact, along with passengers Frederick Wiley and Norman Slavik.
Joe Hincke of the Transportation Safety Board says Williams may have been flying slower than he should have been because of several changes made to the wing shape and floats.
He says there isn't a standard process around assessing multiple modifications to planes, such as those built from a kit.
Hincke says companies must get permission from Transport Canada before making alterations but pilots are not provided with enough information on how that may affect an aircraft's performance.
Note to readers: This is a corrected story. An earlier version incorrectly identified passenger as Frederick Merville.Suggest a correction