Official rule text
45.4 If a player has more than three secret objective cards after drawing a secret objective, they must choose one of their unscored secret objectives and return it to the secret objective deck. This number includes the secret objective cards in the player’s hand and the cards that player has already scored. Then, they shuffle the secret objective deck. RELATED TOPICS: Initiative Order, Objective Cards, Strategic
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 45.4