In a compound cadence in three parts the bottom voice employs a Bass Clausula while the top voices remain the same as in two parts
Notice that the 7-6 suspension we had in two parts is now labeled as 4-3 since we label sonorities from the bass up.
We can create a moving bass by dividing each half note into two quarters. While repeating the Bass Clausula pitches is possible (1-1-5-5), it is common to skip from 1 to 3 and then drop the octave of the bass on 5.
The second example above includes a 1-3-4-5 motion in the bass creating passing motion between 3 and 5. In modern music theory nomenclature, these progressions would be labeled quite differently. The historical terminology corresponds well with the fact that all three are almost identical.
Practice these in all major and minor keys using Keyboard Buddy (unit 1-5)