The Gettier problem @BlueBanana
"The president of the company told Smith that Jones will get the job." Seems like a good evidence, as the president of the company is a reliable source, however, "Jones will get the job" is an assertion about the future and so, it is indeterminate. The president of the company is a reliable source but he cannot predict the future. The mere fact that Jones didn't get the job proves that.
"Smith counts the coins in Jones pocket and sees that he has ten coins. Smith comes to the belief that the person who gets the job has ten coins in his pocket." The problem here is that the person Smith is referring to is clearly Jones because he knows that Jones has ten coins but not that he also has ten coins. He mistakenly thought that a good way to differentiate himself from Jones is to say that, unlike him, Jones has ten coins in his pocket. Therefore, the underlying meaning of "the person who gets the job has ten coins in his pocket" is "Jones will get the job".
Smith believed that "Jones will get the job" which turned out to be false and not "the person who gets the job has ten coins in his pocket" which turned out to be true, so it doesn't debunk the JTB theory.
As for arguments for the JTB theory, well, knowledge is in itself a true belief, the thing is, how can we know that the belief is indeed true? We have to rely on some sort of proof, a justification, hence the true
justified belief.