If I have three LEDs at 3V each = 9V, how can a 5V power supply 9V power to those LEDs? Does it have anything to do with Watts?
LEDs are typically wired in parallel, just like almost every other light on the planet (you are describing a series circuit). 3V LED + suitable resistor = 5V at somewhere around 20-50mA each. Put as many in parallel as your wiring and power supply can handle.
Based on this and some other questions you appear to be missing some fundamental concepts of electrical circuitry. Go find a tutorial on basic electronics, preferably before cooking some expensive lighting fixtures. Books by Forrest Mims are particularly good and available on AMazon.
The 3 volt specification is the forward voltage drop (the diode lights when the anode is more positive than the cathode).
The three anodes are typically connected together and connected (in series) to a resistor. The resistor is connected to the +5V. The anodes are connected together to the -5V (or ground).
The resistor value is a function of the desired current flow through each LED. R = (5 - 3) / 3* LED current
For 20mA LEDs, R = 2/.06 = 33 ohms For 10mA LEDs, R= 2/.03 = 66 ohms
A safe test value if you don't the LED current would be 120 ohms and adjust for brightness from there.
As other respondents have indicated, your 3V LEDs are actually 5V devices, so that jives with a 5V power source.
Watts = Volts * Amps
Watts is "power," or the overall amount of energy used.
They key is what the other respondents have said about parallel vs serial loads.
Water Analogy (Pressure vs Quantity)
You're not really operating from the right premise.
Think of Volts roughly like water pressure, and think of Amps roughly like the size of the pipe the water is running through (or water quantity).
One pound of water pressure coming through a pinhole might get a few drops of water on you. A thousand pounds of water pressure coming through a pinhole will cut a sandstone block in half.
If you pressurize a 1/2" pipe and a 1" water pipe to 40 pounds, you will get twice as much water out of the 1" pipe in the same amount of time, but still at 40 pounds of pressure.
If you attach a water fixture that is designed to operate at 40 pounds of pressure, it will work exactly the same on both the 1/2" pipe and the 1" pipe.
But you can run more of those fixtures (a bigger overall load) off of the 1" pipe because you can push more water through it--more water equals more fixtures--without increasing the pressure.
To push the same amount of water through the smaller pipe would require higher pressure, which might damage your fixtures. Similar (the analogy is breaking down a little) to how if you attach an electronic device designed for 5V to a 24V source, you'll damage that device. With electricity, you'll also damage the device if it's designed for 24V and you attach it to a 5V source.
Parallel vs Series Wiring
The water analogy is useful to this point because it helps you visualize that Voltage (potential, or pressure) and Amperage (current, or the overall "quantity" of electricity) are different things.
Without butchering the analogy any further, lets just conclude with the assertion that if you hook up a bunch of 5V devices in parallel (each load is connected independently to the same electrical bus), the circuit is still a 5V circuit. Each device draws what it needs, and that doesn't affect the "pressure" of the circuit.
However, if you wire the loads up in series (you wire the circuit through each device to the next), then each device still requires 5V individually, and each device will receive the same voltage as the others (the ones "downstream" don't get less "pressure" than the ones "upstream," at least not over short distances and not for the purposes of this discussion). Each device also causes a voltage drop across the entire circuit, so the circuit requires the sum of the voltage requirements of all the devices, in order for each device to get the 5V that it needs. For instance; you need to supply 15V to a series circuit if it has three 5V devices on it. Or you'll get sub-par performance, or damage your devices becuase they'll get less pressure (Voltage) than they require.
But your 5V power source absolutely can supply the correct amount of power to three 5V devices, if they're wired in parallel, and presuming your power source can supply enough Amperage without failing.