I really don't get the idea that there should be 6 in. between the fences. How and who maintains that? In our state fences are shared between the property lines. If both parties have a fence both parties are responsible for paying for the piece that is on the shared line. If only one party wants the fence than they pay for the whole thing. If the other party changes their mind they can hook up to the existing fence but can be compelled to pay half of the cost that's on the shared line.
It is a definitely issue to have them taking part of your property as well as the code issue. I would suggest making an "anonymous" call to the towns code enforcers to come check it out.
I wonder if they would consider you are giving up those feet of your property to them if you do not complain? Would they consider it an easement? Yes, I would say something, because I am not willing to give up 3 feet of my property. If the fence was on the property line I would not care. Seems a cheap way for them to have a fence yard without paying half of the cost. Also, now their yard is 3 feet bigger to enjoy. Not cool.
As I said before its 3 inches setback not 3 feet.
Update: We've knocked on neighbor's door twice - once yesterday and once today - when we saw their cars in the driveway and lights on inside. No answer. They seem to be avoiding us (wonder why?) Unless we see them later today or tomorrow morning before work I'm calling the township.
Can you call or email them? I have emails for my neighbors.
For those who are saying you'd only be upset because the neighbor didn't ask - why? What difference does it make? I guess it would be nice as a courtesy, but I don't see the point in getting pissed if someone doesn't do something as a formality. If the neighbor had asked, would you really have said "no"? If you would, what purpose would that serve? I agree it would look worse/be more of a PITA to maintain if there were 2 fences running alongside each other 6 inches apart. I think it looks cleaner if there is just one fence between the yards.
The neighbor gets the benefit of your fence, true, but that would be the case whether they asked or had their own fence anyway. By putting up your own fence, you're putting up a barrier that affects both yards.
i really doubt i'd say no if they asked, but them asking opens up the opportunity for a conversation about maintenance, what happens if it's damaged, etc. and it's just the right, neighborly thing to do. them not asking makes them seem entitled, shady, unneighborly... which is proving to be quite true if they are, in fact, avoiding mr. and mrs. spunky.