These four-part chorales are still triad chords. The only difference is that:
- Like @replete said, one of the notes (usually the root of the chord) is doubled--For example, in the first chord, there is an
Fin the bass clef and another
Fan octave above in the treble clef.
- The notes are broken into two different clefs. Unlike a triad in just treble clef, the chord's notes are divided, with two in the bass clef and two in the treble clef. This is like this probably to help you learn to read notes in both bass and treble clef.
Just like this first chord in the sequence, you can find the missing chords by figuring out the notes and then finding out what triads contain these notes in the sequence. Using the picture again, the first chord is
F Major. How? The notes
A are the notes that form the triad for
F Major. The remaining note is just another
F, so the chord is
F Major. Again, as @replete said, the doubled part doesn't change the name of the chord.
Hope this helps!