They are 2 year colleges typically teaching some vocational skills or other specialty.
2) 3-year colleges are also a kind of 전문대학교.
Some 전문대학교 have 3 year courses. They are called 2,3년제 대학교 or 2,3년제 전문대학교, as most of them have both 2-year and 3-year courses.
3) 야간대학교 (literally "Night College").
They are for people who work during the day, and have been very common in the past.
In most cases, they are part of a regular 2- or 4-year colleges, not exclusively 야간.
야간대학교 is not as popular as before but there are still quite a few.