Support Multiple Mass Selection in BOM

I am not sure I fully understand your question. I think what you are saying is that your have 3 CAD models which basically represent the same raw material. Am I right in saying that this is a piece of angle stock that comes if various lengths? Here is where the CAD model might diverge from the BOM. In CAD, you would have some unique model of that shape and its desired length. There are infinite variations for different lengths so what to do. 

One solution is as you described is to make a unique model in CAD as needed for each length. They can be linked to the same WTPart to appear on the BOM but they will show up as each. This is where you need to alter the BOM, break the cad linkage to change unit of measure to length. Its a bit of manual work but only need to do it once per assembly.

You could try a flexible model so you only have one model that varies by length but same issue with the BOM. You need to replace each with length at the BOM level. After that, if length changes, you need to manually update it.

It you are publishing attributes from those 3 CAD models, it will make 3 values for that attribute. Be sure to use image link.