とか・そうだ・ということだ: the difference
I heard that… something like, They said ... or something
Tested once in past papers
納豆は体にいいんだとか。
I heard that Natto is good for your body or something like that.
I heard that, It is said that
Tested 4 times in past papers
美味しいそうだから、たくさん買った。
I heard they taste good, so I bought a bunch.
I heard that, Rumor has it that, It is said that, It means that
Tested 2 times in past papers
人によって考え方が違うという事だ。
That means different people think in different ways.
How to tell them apart
「とか」: the least certain; you're not sure, just mentioning it in passing. 「彼、離婚したとか」 (I heard he got divorced, or something).
「そうだ」: the standard hearsay form, reporting what you heard from a clear source, neutral.
「ということだ」: the most formal written hearsay, common in official papers, editorials and news summaries.