I'm studying Andantino no. 15: in 'Studi per Chitarra' by Ferdinando Carulli
but I can't figure out what chords would be under the 3s. I think they are auxiliary notes but since they are on the strong beat and a crotchet long I think they should have a chord or have some harmonic influence to the piece. I'm also confused that one of them is C and the other is C#. They seem to contradict the key signature.
How can these notes be explained and what chords would they be?