What is the chord symbol for measure 3 (e.g.  C, Cm, Cº, C+ )?

The chord symbol for measure 3 is F. the music language has its whole grammar that means there are different symbols in place that need proper understanding if you are looking to learn the entire language. The different components of the music language, including the chords, the symbols and the tones come together to make a specific sound, just like any other language.

The music language has a meaning of its own and you need to learn it in order to make sense of it. For instance, in this example, it is the f major chord. You need to be able to differentiate different chords as well.