Do I need to take all three battlefields to win with Conquest if there is a Baron Pit in play?

Whether you need to conquer all three battlefields to win depends on your current point total.

According to Rule 466.1.b and associated FAQs:

1. If you are NOT at your Final Point

You do not need to control a specific number of battlefields to win. You simply continue to score points by conquering or holding battlefields normally until you reach the Victory Score.

2. If you are at your Final Point (1 point away from the Victory Score)

The rules impose restrictions when you attempt to score your final point:

• If you score through "Hold": You gain the winning point simply by maintaining control of a battlefield at the start of your turn. You do not need to conquer any additional battlefields (Rule 466.1.b.1).
• If you score through "Conquer": You must have scored every battlefield in play (the two original battlefields plus the Baron Pit) during that same turn to gain the winning point.
  • "Scoring" a battlefield can be achieved either by Conquering it that turn or by already Holding it at the start of your turn.
  • If you conquer a battlefield but have not scored the other two battlefields during that same turn, you do not win the game; instead, you draw a card (Rule 466.1.b.2).

Summary: You do not necessarily have to conquer all three in a single turn, but you must have scored all three battlefields (via either hold or conquer) within that turn to win via a Conquer action when you are at your final point. If you can win by Hold, these conquer restrictions do not apply.