I've come across two third cousins, who are third cousins through two different routes as the same two families had two separate marriages between them that leads to this situation.

I was thinking perhaps calling them double third cousins, but would like to know the official terminology.

You can see an example of this relationship here. Notice that there are two sets of married ancestors, the Brown/Hodges marriage and the Brizendine/Cochran marriage, both common relations make them third cousins. See popup menu that allows navigation among the four common ancestors in this screenshot: