Simple, often secret, way of saying I Love You over text or chat, without being too forward

I miss you.

If someone is trying to attack you you say 123 and punch them

used in web forums to denote a quote that they entirely agree with 100%. the term "you took the words right of my mouth" comes to mind.

however, because they forums disable spaming by requiring people to type at least 3 characters per post... 123 comes to mind

the numerical version of "abc".
Which means, "simple"

it is often used as a term denoting the word "numbers" or referred to counting

123 is also the first thing that comes to my mind when I hear the anything related to numbers or maths.

the first numbers you learn in your early childhood.

the number you call (in the uk anyway) for the talking clock that goes on and on...