How do you tell someone's homeroom class in Japan?

Question:

I think it has to do with a combination of numbers an letters like 3-A or something but, I don't know.

Answer:

Using third grade, class 2 as an example...

SHORT VERSION
3-2 (or 3の2 in Japanese ("san no ni")).

FULL VERSION
3年2組 ("san-nen ni-kumi").
(年 represents the year/grade, 組 represents the class number).