1) The driver "pushes" as much current through the load (the LEDs) as what you have set, if you set it to 5 A that current will be 5 A.
However the driver only supports a voltage of around 48 V (actually that's adjustable between 40.8 V and 50.4 V). So with the LEDs connected and the current set to 5 A, the voltage across the LEDs must be less than 48 V.
Suppose you connected LEDs that need 60 V when 5 A is flowing through them. This driver cannot support that, you would get the maximum voltage of 48 V and the current would have a very low value (how much depends on the LEDs).
2) The LEDs are rated for 3 A then you must use a driver that outputs 3 A or less. This driver outputs 5 A, which is much more than 3 A. The LEDs will be overdriven, get too hot and will die very soon. Just don't do this. Underdriving (using a lower current than the LEDs need) is always OK, overdriving is not.
3) Connecting 3 x 3 A LED strips for 9 A and using a 9 A driver will work but is risky.
When one LED fails open: the other strips get 9 A / 2 = 4.5 A and are overdriven. They will soon fail as well.
When one LED fails short: then the other LEDs in that string will take most of the current and will be overdriven. This will make that strip fail sooner. When one of those LEDs then fails open, see above what happens.
It is really a much better idea to use 3 separate 3 A drivers.
