I have a netgear wireless router and live in a densely populated area.
I want to know if anyone other than my 3 computers (all wirelessly
connected, 2 pc one mac) are eating up my bandwith. I do not want to
put encryption or a password on unless i need to, since I find that
causes more problems.

For starters let me say that increasing the security on your wireless
router is paramount unless you like your IP to be used for everything
from DoS attacks to free bandwidth in a 150 foot radius in your
neighbourhood
as far as checking who is using your bandwidth , your router comes
equiped with a tool to do so

go to page 198.162.0.1 and select attached devices , you should only
see your own 3 PC's , if not others are leeching your bandwith