Unable to join surfaces

Hi there, suffering from the age-old unable to join surfaces conundrum!

In the attached model I have three surfaces that each share one identical edge (2nd loft, 2nd surface loft and curve section loft). If I take any two surfaces and join them, it joins the surfaces. However, if I take all three and try to join them it is unable to join the surface. (or if I join two and then try to join the third, same thing.) Why would this be when they all share the same identical edge?

thanks a lot
bodiesinmotion23452560new11.3dm (3.2 MB)

You can’t join them this way, mathematically will result in a non-manifold geometry.
use _nonmanifoldmerge

the result is considered invalid geometry by many applications such as surfacing, meshing although you can use it for renderings.

thanks Tay… I need this file to be 3D printed ultimately, so I need a method to actually join these surfaces

what I’m trying to do is to build out a series of curves into polysurfaces that will ultimately be joined to one closed surface. I want to make an edit to a section of the 2nd loft, but not the whole thing, that’s why I’m creating a 3rd surface

How should I be going about this?

I’d suggest to strategize about building a cleaner geometry, neither rhino will help you nor your 3D printer will like the output of this geometry. try to look into Mobius forms, something like this post.