Entrance light on Lift Master garage door opener won't stay on when door is opened ?

SOURCE: Lift-Master garage door opener Model 1045-6 1/3 hp

There is a thermal cut out in the motor, usually the motor must cool before the opener will operate again. If the opener only operates 3 times it must be heating up too quickly.
First you opener is only made to do your job for you, which is opening and closing the door. If the door does not operate properly manually the opener will not operate properly.
When the door is disconnected from the opener it should lift easilly, it should basically stay at whatever point you let go of it (midway and fully open) and it should close without slamming down. It does not matter what size the door is!!! If it does not something is wrong with your springs and the door can be VERY DANGEROUS!!
With that being said it sounds more like a static lock-up possibly a loose ground or even a circuit problem in the home.
Bryan

SOURCE: Lift-master 1265 Garage Door opener

Open case, & see if white shavings from gear stripped out are laying on bottom of case.

SOURCE: garage door won't open

You probably have a broken spring on your door

SOURCE: Access Master Garage Door Opener opens by itself

The most common cause of this is a bad wall control or the wiring going to it. The wall control goes bad from use and the wiring sometimes has intermittent short caused by the staples the installer used.

SOURCE: Chamberlain Lift Master Pro Formula 1 only makes click sound

Is the door stuck in the open position? If so the trolley is probably jammed against the bolt just in front of the motor unit. Loosen the chain enough to free the trolley and then run the opener until the door is closed. While the door is closed locate the 'up limit' on the side of the motor unit and turn this control 2 complete turns in the opposite direction of the arrow printed on the cover. Tighten the chain.