How to compute the residence time for given flow rate in standard cubic meters?
The N stands for "norm" and should be avoided (in fact, the German Wiki page lists the Nm³ as wrong abbreviation) as it can create confusion.(qed) 

So your flow should be written as "3 m³/h under standard condtions". 
If you can guess German, have a look at the German Wiki page. It is much better than the English one.

Standard conditions vary a bit between different norms 

T = 0 °C or 15 °C or 20 °C 
p = 1.01325 bar (sometimes 1 bar)

The gas is supposed to be dry.