Select the most appropriate idiom to fill the blank.
I'm sorry, I can't come to work today because I'm feeling ______________.

Here are the meanings of the given idioms: 
Jump the gun - to do something too soon 
Once in a blue moon - rarely or not very often 
Under the weather - to feel slightly unwell or ill 
Best of both worlds - a situation in which you can enjoy the advantages of two very different things at the same time 

From the context of the sentence, we can see that 'under the weather' is the correct idiom here.
Hence, the correct answer is option 3.