My vivoactive 3 has just started recording crazy elevations on my runs. I haven't noticed anything too unusual on my walks but my last 2 runs have had 2 of 3 times the actual elevation gain recorded.

There are many possible reasons - first of all make sure you regularly calibrate the atimeter, or use the auto-calibration. Then also carefully rince and dry the watch after each use, to avoid that the pressure sensor hole gets clogged with sweat, dirt, and dust. Keep the pressure sensor oriffice dry and clear of debris during the activity too (I dry it frequently wth my t-shirt, when sweating). When the weather is windy, or you make fast bike rides, protect the sensor from the wind gusts (let it under a sleeve, use a sweat-wristband, etc.).

Finally, when, despite all precautions, the barometric altimeter still records wrong elevation data, simply go anytime to the concerned activity screen on Garmin Connect Web (not the GC Mobile phone app!), and enable Elevation Corrections - the elevation profile will be recalculated, using the topographic data instead of the barometric data.