Is rebooting the computer normal before/during flights?
It is wrong to write about "the computer" because a modern airliner probably contains hundreds or thousands of microprocessors and microcontrollers.
For example, the seat-back information and entertainment systems almost certainly each contain a microprocessor running some embedded operating system. Each of the items of avionics equipment will contain several microprocessors, probably running real-time operating systems that are certified for use in aircraft.
Critical avionics systems have the ability to reset themselves if they detect problems - for example they will have hardware-based watchdog timers that will restart a processor or system that isn't showing signs of running it's software normally.
Unlike a typical desktop computer, I'm pretty certain that critical systems on board an airliner are designed to handle multiple failures and keep working, in a degraded or fall-back mode if necessary.
most planes have 3 computers
As I wrote above, large commercial airliners will likely have hundreds or thousands of microprocessors and microcontrollers.
The "3 computer" idea comes from the use of redundancy in each critical system. Each subsystem may have two, three or more independently created systems with different hardware and with software written by different teams, each doing the same job. An arbitration system compares the outputs and if one of three disagrees, the other two get to determine what happens.
No, it's not normal part of the pre-flight checklist for the pilot to walk round to the rear of the aircraft, flip a big red switch off and on again and wait for a start-up jingle.
If the in-flight movie system isn't working, I'd expect someone will try turning it off and on again.
If there is a problem with one of the many independent systems in the cockpit, I'd guess it's plausible, in some cases, the pilot or engineer's checklists for the problem include some kind of reset operation.