Travelling Tokyo, Kyoto, Osaka - What rail pass do I purchase?

Hi Daniel San,

I am travelling Japan for around three weeks with three of my friends.
During the three weeks, we will be visiting Tokyo for seven days, Kyoto for four days, Osaka for seven days, then back to Tokyo for six days.

We are not sure what kind of rail pass to purchase, weather we only get a 14 day pass and stay in Tokyo for either of the times we are there or purchase a 21 day pass.

Thanks, Georgia.