You score 5 victory points when you complete a Community Building.
Afterwards when I complete another Settlers Card am I getting those 5 VP again?
Victory points that are covered up by a building are awarded once upon being covered up. And if you sell that building, removing it and uncovering those VPs, you deduct them from your score again.
If I were to complete say two Settlers Cards to gain workers in order to then complete all three Community Buildings, for each Settler I complete afterwards would I then score an extra 15 VP for the 3 completed Community Buildings?
No. You would score the normal VPs that you get from the settler cards based on the goods you put into it, +1 VP for each community building that is already built on your board.