They're all correct. Nether is better or more polite than the other.

The last one is correct.

i think the last one is better

Honestly; "Where's the point talking about that now?" Is incorrect. We normally sy "Where's the point in talking about that now?" It sounds more complete. "What's the point of talking about that now?" Is entirely correct. "There's no point talking about that now." and "There's no point in talking about that now." are both correct and they both make sense :) All of these sentences, mean the same thing. They are equally correct and equally polite. However, in my opinion, "There's no point talking about that now." and "There's no point in talking about that now" sounds slightly more rude. :)

Where's the point talking about that now? and Where's the point in talking about that now? are both incorrect! The only correct way is What's the point of talking about that now? No There's no point talking about that now. and There's no point in talking about that now are both correct and mean exactly the same thing!