Yes, it is s, in Morse code as ●●●.
A mnemonic to help you remember is to imagine a letter s. Now put a dot at the letter's top, middle, and bottom. Then, topple the letter over so that the dots have a horizontal orientation, and fade out the s.
Some practice:
●●● a c k
b a l l ●●●
●●● ▬ ●● n k
● a r ●●●