Operation of the try cocks was as follows:
Open the top one, if the water level was below it, you got only live steam (invisible) out of it, appeared like super-hot air.
Then you worked down to the lowest one, when you opened one below the water level, you got water which flashed into steam instantly, and was visible (appeared as wet "steam").
Absolutely no doubt when you opened one below the water level.
These were more of a "coarse" water level measurement, not as accurate as the gauge glasses themselves. Older engines had a gauge glass on the firemans side, try cocks on the engineer's side. I Believe the boiler rules now allow two glasses instead of one and a set of trycocks.
Engines that were used on steep grades where the water level was subject to lots of variation due to the engine going up and down
hills often had four try-cocks instead of the commmon three on one side.