Meaning of Conditional sentences with 'should' and 'happen to'

I'm a bit familarized with conditional sentences. But the following sentences with 'should' and 'happen to' are quite confusing too me. They are:

1. If it should rain, I will stay at home. 2.If it should happen to rain, I'll stay at home
   
   
   
   
   3. If it happens to rain, I'll stay at home.
   
   
   
   

What are the meaings of those sentences ? Do they all mean the same ? Thank you a lot in Advance for your answer.