Shorted that could be 40 volts at 3 amps. That could be done with one transistor on the infinite heatsink. I suspect they used three transistors to allow a smaller heatsink and still allow for a short. (Yes there is some other protection in the circuit for that.
Yes but then he didn't show the value. So matching the transistors may still be needed. If the thingy is old enough the transistors could have had a beta as low as 10 under load. It is unlikely they would have reached 50!
I would not use a plastic version of the 2N3055 even though it should work well. The reason is that all modern 3055's far exceed the published specs! I would look at the TIP family. Cheap slow and low gain by today's standards.
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