When trying to determine the polarity of a chemical species its important to consider the molecular and electronic geometry of the species and try to map out where exactly the the dipole moment should be (if it exists).
Consider the graphics below:
You could draw a lewis structure for
But it's important to realize that it actually looks much more like this geometrically:
With the lone pair on the central species N significantly affecting the polarity of the molecule.
The nitrogen within the molecule is more electronegative and the valence electron about the species will be closer to it than to the hydrogen.
We sometimes write this as
Here are the simple lewis structure for the other species:
I recommend to look at their structure and try to think about the polarity of the species.