Is the 100w LED I have really 100w?
Well, there's your problem:


  I adjusted the voltage booster to output 34v exactly (measured with my
  multimeter)


Look - on the one hand, LEDs (even the same model from the same manufacturer) do not all have the same voltage vs current curve. Assuming that 34 volts is an exact value will cause you nothing but heartache.  On the other hand, attempting to drive a bare LED (without some sort of current limiter) from a voltage source is really great way to kill an LED.

The problem is that LEDs have what's called a negative temperature coefficient. That is, for the same current, as the LED gets hotter its voltage drops. Put another way, for a fixed voltage the current will increase as the LED gets hotter. So let's say you have an LED connected to a voltage source. At first, it will draw some current value, and dissipate power appropriately. In the process, this will cause the LED to heat up. This will cause it to draw more current, and therefor dissipate more power and get even hotter. Depending on your cooling system, it's entirely possible that this will form a vicious circle which eventually burns out the LED.

So don't do it.

Instead, either use a current source or put a current-limiting resistor in series with the LED. Figure on dropping something like 10 to 20% of the LED voltage in the resistor, if that's the way you go.

Now, as to your specific current. You have measured the converter output voltage - but did you do so with the LED connected? Having measured the LED voltage, why didn't you measure the current as well? If you do, you'll be able to calculate the LED power directly, rather than trying to back it out of the converter input numbers. And you should be aware that any voltage converter will have an efficiency less than one. That is, if the input power is 100 watts, the output power will be less. It depends on circuit and load, but 85 to 90% efficiency is a pretty standard number. Problem is, you don't actually know what yours is.

So. First, change your LED circuit. You want to set the converter output to several volts greater than the nominal LED forward voltage, then add a resistor to limit the current to a reasonable value. Second, measure both LED voltage and current, rather than messing around with the converter inputs.

In your particular case, you got lucky. Your LED actually needs more than 34 volts in order to pull 3 amps, so you wound up not killing your LED.