Start with a Rectangular Beam and investigate it, you will see it is made of a Sweep with a Rectangular Profile - Save the Rectangular Beam as a new Name as not to edit/change the original.
Select the Profile from the Project Browser > Edit
VG > Turn ON required elements for editing
You then need to decide which edges and offset are driving the position of the Side Faces!
I copied the Left Hand Reference Plane and Delete the EQ constraints at the Top
I then added a b2 Parameter to diver the Top Face Width
Then moved the Lines to the Intersection of this point and use the Align Tool to Lock the Lines to this location
Then Load it into the NEW Triangular Beam Family (Save As)
Edit the Sweep > Swap Profiles
Select the Triangular Profile in the Project Browser > Right Click Type Properties > Map the Properties from the Beam Family into the Profile - Add an Additional Parameter for b2 using the Icon in the Bottom Left of this dialogue
If you want it off-centre you will need to add Parameters for the EQ dimensions and apply them as required
Thank You very much for Your help.
Best regards.