3Nm = 2.2ft/lb so a 6" spanner would need a force of 4.4lb to give about 3Nm
I haven't looked the numbers up, but quick sanity check in my head makes this about right (although the words "sanity" and "my head" do not normally appear in the same sentence).
Luggage scales I think are possibly too inaccurate for your purposes. I would use two 1kg bags of sugar / flour hanging by a length of string myself; I would check them on a set of digital kitchen scales first.
This is probably teaching you to suck eggs, but remember to make sure that the force applied is at right angles to the spanner, e.g. if you are using a weight the spanner must be horizontal. The string holding the weight has to be six inches from the centre of the bolt.