Since there is no more Fenix 3 HR forum, I will post here!
With the 5.40 update (even earlier) you've just broke the 3 HR! Every time it's connected to Garmin Connect IQ, the watch just restarts continuously, the altimeter it's bouncing every minute (+/- 10m), the battery drains much faster (watch is new, purchased in APRIL 2019). Had the Fenix 5x Plus before, had to sell it because I was at my 3rd one, 2 of them changed under warranty because of altimeter/barometer/compass not working and screen issues, you don't have to "convice" me to upgrade from the 3 HR by ruining it with bad software updates! Had the Fenix 5, then the Fenix 5x Plus, but I preferred to go with the Fenix 3 HR, less problems with it, until NOW.
Hopefully you will adress this issues, fix them, if not, I will gladly NOT buy any other Garmin product.