Primarily asking for the WR, LB, and CB positions. Specifically, are the first two or three WRs starters? Are the starters different in a 4-3 versus 3-4 defense? How many CBs are starters?

The starters are the players that are on the field for your first offensive and defensive play. So if you come out in a 5WR set, your 5 WR's will get credit for the start and your backfield won't.

So for the pull starters option in preseason, it's difficult to tell who exactly will be considered a starter beforehand?

Oh, sorry, I thought you meant for the starter stat.

The pull starters are the #1 in all positions except WR and CB, where it's #1 and #2. They will only get pulled if there are enough players behind them in the depth chart (i.e. if you only have 2 QBs in your depth chart, the #1 QB will not get pulled because there will then be no backup QB)