Question

Alone I’m unknown, With my twin I’m twenty, And triplets make me a warning. What am I?

Answer 1

${\huge {\mathfrak {\underline{\underline {Answer}}}}}$

The answer is the letter "X"

In maths it is often used to indicate something unknown. With two (XX) It means 20 in Roman numeral. And the three X is used to say warning that the content is related to adults only.