How to drive three 3W LEDs from a USB power supply (5V/1A)?
It's possible to use a boost converter for generating 12V, and it's also possible to use several of them in series.
But it doesn't help in your case, because you can't generate 9W from a 5W power supply.

Consider also the efficiency of the booster. They promise "up to 94%", but I would rather assume about 80..85%*.

Next thing:
The booster in your link generates a constant output voltage. You can drive the LEDs with a constant voltage converter, but especially for High Power LEDs I would recommend a constant current converter, as the forward voltage is temperature dependent.

After all:
If you are sticked to the power supply, use a constant current buck driver and drive only one LED. 
If you need the power of more LEDs, then use a stronger power supply (and nevertheless switch to a constant current driver).

*) In a series connection of several boosters, you would have to multiply all efficiencies, so the overall efficiency was much lower.