Vertical or horizontal assembly

[thereiwas]

I wonder if SpaceX plans to assemble a Falcon-9-heavy horizontally? I don't see how - it has 3 Falcon-9 first-stages side by side.. Perhaps each of the three first-stage components could be done horizontally individually, and then erected and fastened together at the pad. I have not seen any details about this at their web site or in the F9 data sheet.