Ok, there are rules the NFL has in place. One of them is that any player can be extended after their 3rd year. There is also a significant advantage to extending a player after their 3rd year, not 4th, and not when they become an FA. Even if they have a 5th year option. After the 3rd year is ideal for any team.
For example. If they don't extend Lamb or Diggs after next year then it's because they either have doubts, or they plan on trading them or letting them go. Not for any other reason. Mahomes, Watson, Allen, Wentz, Goff, all got extended after 3 years. QB's like Daniel Jones, Lamar Jackson, Kyler Murray, are not extended because the team has doubts. I think Diggs gets extended after this year and Lamb does not unless he has a breakout year.