rain

i'll try to help,

the first and the third are similar, we can say that "to rain" is working as "raining"..

so the first and third are present perfect tense..

but the second one is present continuous tense..

I know the tense is different but I don't see the effect of the tense difference on the timing of rain. Could you please elaborate? Thanks in advance!

The timing is VERY close.

You look out the window and see those VERY FIRST drops or the pavement just starting to get wet - it's starting to rain.

You look out the window and it's raining, and the pavement is just about all wet, but it's not coming down hard (yet!) - it's started to rain.

And in truth, you can use either one in either situation without sounding too weird

GG, thanks for the detailed explanation.