Hi Andrew,
This is indeed current limitation with FormIt's modeling engine.
But it's something we can fix in a future release, and we'll make sure your request is heard to extend the fillet tool to support 3 simultaneous perpendicular edges.
In the mean time, you can accomplish this in FormIt with a workaround:
- Make 2 copies of the geometry you want to fillet (let's assume a cube per your screenshots)
- On copy 1, fillet 2 edges, like the top two in your screenshot
- On copy 2, fillet the 3rd edge only, like the vertical one in your screenshot
- On copy 2, select the new fillet curved surface and copy it
- Paste-in-Place (Ctrl + Shift + V) the curved surface, then move it (using an edge of copy 2 as a reference point) to the exact same spot on copy 1. This should split the surfaces on copy 1 automatically, and you can quickly clean up unnecessary geometry using window/area select and delete, leaving you with 3 filleted edges.
Hope this helps until the fillet tool can do it for you.
Josh Goldstein
Product Lead