Rainfall is a noun. It has two meanings:
1) the amount of rain that falls in a particular place and
2) a fall or shower of rain.
Taken from the New Shorter Oxford English Dictionary.
Rainfall is an uncountable noun. So, you wouldn't use rainfalls. We talk about a measurement of rainfall and not a measurement of rainfalls.
"Rain falls" is a subject and a verb, e.g. rain falls in the winter.
It is difficult to say which of the phrases you mention is correct because they are all taken out of context. Without context, language sounds strange. Based on my explanation above, some rain falls is correct and the others are incorrect.