Have you used components or groups for the various pieces of the joint? If not, you will need to do so, and it may require some careful selection to get the appropriate edges and faces without also getting others. I started over, assuming 1x2 aspect ratio on all three main parts. Created one and then positioned the others by move copy and rotate copy.
Your profile says you are using Make 2017, which means you don’t have solid tools available. So, you will need to use right-click Intersect Faces with…
First create a face sloped in the correct orientation and aligned with the corners of the joint. It can be a triangle joining the appropriate points. Cut this face to the clipboard, open a component for edit, and do Edit->Paste in Place. Then select the face and the portion of the end of the component it will cut and run Intersect Faces with…selection. Finish by erasing the stuff beyond the cutoff. If you used the same component for the three long members, all three should now be cut. You can open the little cube at the corner for edit and repeat the process to cut it.
Pay attention to reversed faces, and fix them where they occur.