Parametric 3-way joint

thingiverse

You can find a helpful 3-way joint tutorial on that reddit thread, which features a printable part suitable for 2nd layer attachment, provided in two matching halves due to identical design. To assemble the joints successfully, thorough sanding might be required post-printing depending on how well they align, with tighter fits potentially benefiting from finer adjustments and looser connections requiring slightly coarser sanding to achieve proper interlock.