Hi,
Your power supply puts out 30v max and 2 amps max. That's 60 watts. If it put out 3v at 20 amps you could run a lot of LEDs in parallel with small resistors in series if they all took say 2.5 volts each.
But your supply does not put out 3v at 20 amps, it puts out 30v at 2 amps, so you've got to change the arrangement of LEDs if you intend to use the full power of the power supply.
To do this you create strings of LEDs that can run with a single relatively small value resistor, then connect the strings in parallel.
Now if the voltages of the LEDs were perfectly 3v all the time, you could run 10 in series connected to the power supply, and that would draw 300ma. Since you have 2 amps available and would then only be using 0.3 amps, you still have 1.7 amps left. 1.7 divided by 0.3 gives us an integer 5, so 5 more strings can be connected. That would be a total of 6 strings of 10 LEDs in series for a grand total of 60 LEDs. Now 60 LEDs times 3v times 0.3 amps gives us 54 watts, so that's close.
But the world isnt that perfect, the LEDs wont be exactly 3v and there could be a variation with temperature and other factors, so we only use 9 LEDs per string instead of 10 and use a dropping resistor to drop the voltage a little and help to ensure the right current goes through the LEDs.
Since 9 times 3 is 27, that leaves 3v to drop at 300ma, which means we need a 10 ohm resistor. 3v at 300ma is 0.9 watts, so we use at least a 2 watt resistor. We might also check to see that with that 10 ohm resistor we get the required current.
But in doing that we lost 6 LEDs, so what if we use 7 strings instead of 6 and underdrive the LEDs slightly.
2 amps divided by 7 gives us 0.2857 amps. Thus, we would adjust the current for each string to this value and we can use 7 strings with 9 LEDs in each string. That's 63 LEDs but since the current is slightly less we still end up with 54 watts worth of LEDs.
Dropping 3v at 0.2857 amps means we need a resistor value of 10.5 ohms, and power dissipation is still close to 1 watt so we should use a 2 watt resistor or better. A 10 watt resistor here would stay cooler though and those sizes are quite common. More than likely though we could get away with a 10 ohm resistor but if we have to we increase it slightly with another smaller value resistor of 0.47 ohms at 1/4 watt.
But things are not always this perfect either, so the 10 ohm resistors might have to be adjusted in value a little unless they provide less current than we wanted, and then we can simply put up with the slightly dimmer light levels.
Obviously if you want to use an LM series three terminal regulator to regulate current you'll have to use one device per string. Theoretically you can use one LM device total for all strings, but if one LED in one string blows open, that will force more current through the remaining good strings which would then stress them too much and probably overheat them. You also might have to drop the number of LEDs in each string so that the LM device has sufficient overhead voltage to work with. If your power supply is well regulated you might get away with 9 in each string.
So that's one way to go about it.
You also want to check to make sure that your power supply can put out 30v at 2 amps continuously.
Also, under driven LEDs have much longer life so you may want to drive them at somewhat lower current levels like 250ma or even 200ma perhaps. The life goes up quickly as the drive current drops it's not even close to proportional. You might have to check for noticeable color shift however.