Yes, today it is the letter o, that is ▬ ▬ ▬ in Morse code.
A mnemonic you can use is imagine a circular letter o that is cut into three curves (reminds me of how the circumference is about three diameters long), and the curves separate and flatten to get you ▬ ▬ ▬.
Now practice:
▬ ▬ ▬ g r ●
c ▬ ▬ ▬ c k
▬ ▬ ▬ ▬ ●● l ● ▬
b ▬ ▬ ▬ g