The distance between the place a person is and where Jumuah prayers is going to be held should not be more than 2 farsakh (11 Km) and it would be obligatory for a person who is at the end of 2 farsakh to join the Namaz. And similarly, participation in Jumuah prayers will not be obligatory for a person who finds it extremely difficult, because of rains, severe cold and so on'

Does it mean if I leave within 11km and I have no excuse I must attend Jumah prayers or does it mean I must attend 1 in 3 jumahs or it’s not totally must until imam reappears?

This applies to the era of the Imams (a). If the Prophet (s) or Imam (a) leads the prayer, then it’s mandatory upon all men who live within that distance to attend the prayer. But in the time of Ghayba, it’s not mandatory. It’s optional. Even if you live close by, you are not required to attend.