It’s a rescue firefighters hope they never have to make: A colleague injured, trapped or unconscious, cut off by fire and smoke in a building collapsing around them. Over three days this week, firefighters across Quincy will receive training to prepare for the particular challenges they face when a fellow firefighter goes down during a fire, a scenario that officials say is rare but potentially deadly. In the first day of training on Wednesday, several firefighters practiced techniques for moving an unconscious firefighter through mocked up windows inside the vacant rooms of the former Quincy Medical Center. “It’s training we do about once a year,” Deputy Chief Timothy Burchill said. “It’s one of the most important jobs on the fire scene to know how to do.” Burchill said that when a firefighter goes down and is unconscious or has a medical emergency, it’s called a mayday, and fortunately doesn’t happen often. Mayday firefighters are often saved by the crews they go into a fire with, but a rapid intervention team can be sent in as the last resort to save a firefighter.