The only way I have found to create a triangular prism in FlexSim is to start with a plane visual and change the visual display to column and then change the number of divisions to 3. However there does not seem to be a way to create one in the flow items. It seems strange to me that I can do this with visuals in the model but can't make a flow item.
[I am working with on a model of a manufacturing cell dealing with triangular steel tubes so it would have been easier to do this within FlexSim instead of having to create one elsewhere to import.]