Garmin Vivoactive 3 Questions & Answers

The heart rate display has been going haywire, I can be sitting inactive for awhile with a heart rate of 70. I get up and move and suddenly my heart rate is 110+. I then check myself with a blood pressure monitor and find I have a heart rate of 70. Yesterday I rode my bike for 23Klms and when I arrived homely watch was showing 150 checked with BP monitor and I was 114. Is there anyway to calibrate ?

Can the Vivoactive 3 sync with pre planned routines that you set up in the Garmin app. eg if i want to run 8 x 400m reps with 90 seconds rest, can i set that up in the app,then use the Vivoactive 3 to track that session or me?

Can the Garmin Vivoactive 3 HR be paired with a chest monitor strap?

How does everyone rate the Garmin Vivoactive 3 compared to the Samsung Gear Sport 3?