I heard they got their name from either something in their hometown or 311 being a police code for drug possession or something like that...
Editted to add from 311 website:
6. How did the name "311" come about for the band?
mshollan@indiana.edu, Cory328, Limpy311,
rwalsh@umich.edu (Ryan P. Walsh)
311 is an Omaha police code for indecent exposure. One rainy day, P-Nut and some friends went skinny dipping in a public pool. They were apprehended by police. One of P-Nut's friends (Jim Watson) was arrested, cuffed (naked) and taken home to his parents. He was issued a citation for a code 311 (indecent exposure). We thought this was funny, so we took it as our band name. After the humor of the name wore off - we still kept it because we liked that it was just abstract and that it did not define us in anyway. The name did not describe our sound or our politics, it just let the music speak for itself. Since most interviewers always ask us "What does 311 mean?", we have come up with lots of different answers over the years. Some include.... Nick - "five friends making music", Tim - "a number dictated to me by a higher intelligence", P-Nut - "knowing a little numerology and studying a little magic, which I do; in some factions, three is man and 11 is magic. So 311 is like male magic."
http://www.311music.com/htmlsite/qanda/index.html