"Dime a dozen" is an idiom used to describe something that is extremely common, plentiful, and therefore not particularly valuable or special. It originally referred to goods so cheap and abundant that a dozen of them could be bought for just one dime, but the phrase is now used figuratively for anything ordinary and easy to find, including people or ideas.
Options 1 and 2 take the phrase completely literally, treating it as an actual pricing statement about dimes and dozens, rather than recognizing it as a figurative expression about abundance. Option 4 states the opposite meaning, difficulty in getting people, when the idiom actually conveys that such people or things are easy to find because there are so many of them.
Since the idiom conveys commonness and easy availability, option 3 is the correct meaning.