Potential scenario why you can't count on a 3rd round comp pick for Fuller:
Hypothetical Situation
1. Fuller signs as a UFA in 2021 to another team for $12.5m APY
2. Among other FA's this ends up potentially ranking Fuller as a low end 3rd round or high end 4th round
1/?)

3. Houston signs a player, let's say Josh Reynolds, for $8.5m APY. Reynolds compensatory value is a low 5th.
4. Fuller has a season ending injury week 2 of the regular season.
5. Reynolds has a decent season, playing 82% of offensive snaps

There is a real chance that Fuller's back end 3rd round comp value drops enough down to a 5th, coupled with Reynolds high snap counts. Causing the two to cancel each other out.

Lesson here: Don't count on comp picks.