The characters changed over the course of the series.
In the beginning Elaine was clearly the poorest of the the three (I don’t count Kramer).
When George is selling real estate and Elaine asks him to find a flat for her, he notes “It’s hard to find something in your price range.” Elaine also has a roommate(s) at first.
This changes as the series goes on. She reads manuscripts at the publishing company. This is about a $35,000 - $50,000 a year job or less, depending on if she does editing or such. These are today’s range so back then it would be less. (From Salary.com)
Elaine’s job must be entry level 'cause she gets George a similar job.
When Elaine gets a job with Mr Pitt she is his personal assistant. This can pay pretty much anything. But it appear Elaine is slightly better off with this job than the last one.
Elaine went to Tufts. as it was her safety school. We assume she graduated though it’s never said.
By the time she takes her job with J Peterman, Elaine’s income is pretty much up there, even before she briefly takes over job as company president.
George is a college graduate and says so. When he is out of work his parents urge him to take a civil service test and George says something along the lines of “Im a college graduate and you want me to be a mailman”? But knowning how George lies…Well we’ll take his word for it.
George is doing OK at real estate though he clearly is near the bottom at his company. When he goes to the Yankees he is the assistant to the traveling secretary.
But during this time he is mostly with Susan so is she paying his bills? Susan makes much more money than George and her family is loaded. Though after her death this seems to surprise George, which I don’t know why it does. He is some sort of executive in his next two jobs. George has established himself as middle class.
Jerry is a working comic, and has much more money than any of his friends. Jerry either has a great deal of money or he doesn’t care about money. Like when Kramer leaves his door unlocked and his stuff gets ripped off. He doesn’t seem that concerned. Or when Jerry destroys the washing machine he is not all that concerned at the amount of money he has to spend to fix it. (Yeah I know he broke it). He spends a huge amount of money on a jacket. All these occur pretty early in the show.
Kramer is a joke character so other than “falling into money” as George puts it or suing people or his coffee table book, we don’t know much about him.