What does this power supply label mean? What can I replace it with?
LEDs tend to be specified as being fed by a constant current because of the unpredictable voltage characteristic: -



If you studied the above you would conclude that as you turned the voltage up on this particular LED, current would start to flow at about 1.7 volts and by the time you got to 2.1 volts this particular LED might have fried.

Of course there are plenty of different LEDs available but they all tend to have this sort of characteristic so, it's easier to control the current rather than try to predict what forward voltage is the "Goldilocks" value. It also changes with temperature so it's better to control current.

What your LED driver tells you is that providing the load has certain characteristics in terms of its resistance, 300 mA will be delivered. If the LED load resistance is 80 ohms then the terminal voltage will be down at 24 Vdc. If the LED load resistance is as high as 153 ohms then the driving voltage will be 46 Vdc and, importantly, if the LED load resistance rose above 153 ohms, the driver output will not be able to supply greater than 46V in order to keep the constant current of 300 mA flowing.


  What would you replace this with to power 3 of these over a long
  distance? 48V 5A?


Replacing three of these to power 3 LED units is the wrong way to look at it. For a start you'd need 3 x 300 mA but the problem arises when one LED fails and the other two share 450 mA each. You could wire three LEDs in series and feed 300 mA but that then requires potentially a supply voltage of up to 3 x 46 volts (138 volts DC).

You might be able to get away with non-armoured cable providing you use an RCD at the AC feed point from your house after all, fish ponds only use rubber cable and they go underwater. You could bury some trunking to put the rubber cable in.


  Any chance they'd be dimmable?


There is no guarantee that these devices are dimmable.