Venting by gas meter
I called the gas company (I wasn't able to originally when posting the question as it was Saturday) and they said that the 3' is from any side of the meter - so essentially a bubble around it. As long as the vent itself clears this, it'll be fine.

I'd also called the tankless water heater manufacturer located in the US, but they said that this could vary by gas supplier. So although my gas company allows a 3' bubble, others might be more like the Canadian code and require 3' on either side up to 15' above.

Just for reference as well, the gas company was also able to look up the BTU output of my meter from within their system to tell me how much I could load it down with.