Issues with stacking shipping containers at odd angles?
I suspect you'd do better to support the upper level on columns/posts, rather than directly supporting on the containers below. I also suspect that you're engaged in fuzzy thinking if you think 3 40x8 foot boxes will make a nice triangle, unless you REALLY start messing with the structure. 90 degree ends don't make good 60 degree corners.

I think you'd likely be better off building from scratch if you want the shape you describe. It's not well-suited to shipping containers, and by the time you cobble things together to make it work with shipping containers, a ground-up build will probably be ahead, and far less compromised.