JR Pass yes or no?

Hello,
we are a family of 4 and will arrive in Tokyo Narita on may 15th and we leave Japan may 30th, also Tokyo Narita.
We will travel from Tokyo to Hiroshima, from Hiroshima to Kyoto and from Kyoto back to Tokyo.
Would it be a good idea to buy the JR Pass (14 days) for these 3 trips, or is it better to buy seperate tickets.
Is it possible to use the JR Pass to get from Narita to Tokyo and back to Narita?
And last question is - how far in advance is it possible to buy the JR Passes?
Thank you very much for your help.
Greetings from Germany
Joerg, Florence, Chiara, Romane