Understanding constant current LED drivers (ac to DC)
The specifications for this LED driver (300 mA, 36-63V) means the following:


You must use LEDs (or LED clusters, whatever) that are specified for 300 mA nominal current;


and


You must use a number of LEDs in series, such that their total forward voltage (Vf) is above 36 V, but less than 63 V worst case (usually at cold).


Then the LED driver will drive the chain of LEDs to their sum of Vfs, which should be between 36 and 63 V, with constant current of 300 mA.

If Vf of your chain of LEDs sums up to 60V, this LED driver will work to its maximum capacity of 18 W.

In short, "wattage" of LEDs has little meaning without knowing their Vf. If you say you have a LED lamp of 6 W at 300 mA, it means that nominal Vf is 6/0.3 = 20 V (internally there are several LED chips in series). So yes, connecting 3 of these LED bulbs in series will sum up to 60 V and be about optimal. This answers Q1.

For Q2, yes, two bulbs make Vf at 40 V, which also meets the driver specification. The driver will drop its output to 40 V while maintaining 300 mA drive current. They will work just fine, and the light fixture will consume 12 W (plus losses).

For Q3, it depends on driver design. The 20V is below the CC (constant current) capability of this driver. The result will be either intermittent flickering, or reduced (but steady) current. Here is the load characteristic of some MeanWell driver:



Red region shows unpredictable (design-specific) behavior of the driver, but the current shouldn't exceed 300 mA in any case.