Where do Opposition cards go at Setup?

The rules say to do this during Setup:

Finally, the players choose one Opposition force to face. They shuffle the Opposition deck and play the top X cards of the Opposition deck across the 3 sectors, where X = the number of players plus 3. The remaining Opposition cards, as well as the Opposition Flagship panel and token (or mini) should be placed to one side for now.

Do you put them all in one Sector?  Do you have to make sure they are distributed evenly?  Is there something on the cards which tells you where they need to go?

You distribute them evenly among the 3 Sectors. You cannot look at the Oposition cards first and decide which will go where, but you can decide beforehand which Sector will have more cards (if they don't divide evenly by 3).