When characters take off their glasses, why do their eyes turn into a "lip eyes" icon?
The "3" is a stylized (and humorous) way of showing the characters squinting their eyes, which then look somewhat like a "3".

Most glasses wearer &mdash; especially the young ones &mdash; are short-sighted, so whenever they take off their glasses, they start squinting. The "3" is used in a similar fashion to depict pouting characters, like with the following emoji:

(°3°)

A variation of this is the famous へのへのもへじ (henohenomoheji), which uses Hiragana characters that look somewhat like facial features and is used almost exclusively for humorous purposes:


Image taken from RocketNews24