In music, C1, C2, and C3 refer to three different octaves. Among them, C1 refers to the pitch with a frequency of 261.63Hz, which is middle C (i.e. do) on the piano, while C2 refers to the pitch with a frequency of 523.25Hz, and C3 refers to the pitch with a frequency of 786.5Hz.

In simplified musical notation, the pitch of notes is represented by do, re, mi, fa, sol, la, and si. If you want to represent C1, C2, and C3 in numbered musical notation, you can express them as follows:

C1: high-pitched do (that is, add a dot to the high-pitched do)

C2: alto do (that is, add a dot to the middle do)

C3: low do (that is, add a dot to the low do)

It should be noted that the pitches in the simplified musical notation are relative at middle C, so if you want to convert the pitches of other notes to the pitches in the musical notation, you need to make corresponding adjustments based on the position of middle C.