The first sentence would be extremely hard to understand for any native german speaker.
"Es gibt viele Sachen, über die wir sprechen müssen" would be the correct way to phrase the sentence
The other two seem perfectly fine to me. ^^
Edit: Whoops, seems like I totally overlooked a mistake here. In the third sentence "welches" is wrong, "welchen" would be the correct form to use. Thank you so much for pointing that out DonHolgo, I should read more carefully!