You do. It takes time to learn. I think HR is perfectly fine for stuff around 8min and up. I still tend to disagree for the 3min stuff though. Just my opinion.
If the HR for 3x3's reaches target on the first rep, you went too hard for the first rep. If you subsequently complete the rest at the same target HR, you likely decreased your output for each subsequent rep and set.
3x3's are 1:1 intervals, meaning it winds up still being fairly aerobic. If you're taking 5 to 10 minute breaks between each rep......that's not a 3x3. That's just 3min anaerobic hill sprints. Even less useful for HR as it's just all out.
With 3x3's, the ending HR should gradually increase over the reps and sets either until completion of the workout.......or failure.
In this case for me.........it was failure on the last rep:
https://www.strava.com/activities/2495353427