By "service box" do you mean a subpanel? It's been discussed to death here that you'll need four conductors. I think the triplex is OK inside conduit, but you need to add another insulated conductor so you have separate ground and neutral. The neutral conductor needs to be the same gauge as the other two insulated conductors.
Installing a large conduit in an attic seems like a lot of aggravation to me. But it's your time and money.