Final answer:
The three letters that can signal a /s/ pronunciation for the letter c are e, i, and y, referring to the concept of a soft c in English phonics.
Step-by-step explanation:
The three letters that could signal that c is pronounced /s/ are e, i, and y.
This makes the c soft.
When a c has one of these three letters right after it, it is called a soft c and spells the /s/ sound, as opposed to a hard c, which spells the /k/ sound and typically occurs before other letters.
For example, in the word 'cent,' the letter c is followed by e and is pronounced as /s/.
Similarly, in 'city' the c precedes an i, and in 'cyst' it is followed by y, both of which result in it being pronounced as /s/.