Apple has not advised of any changes to heart rate sensor design or performance for Series 3 vs Series 2 models or under watchOS 4.x vs watchOS 3.x.
Apple advises that, even under ideal conditions, Apple Watch may not be able to record a reliable heart rate reading every time for everybody.
For the best performance from the heart rate sensor during workouts, ensure that you wear your watch snugly (but not too tightly) on the top of your wrist. Apple suggests that you consider tightening the band before starting a workout and loosening it again afterwards.
The sensor is likely to give better results for workouts that involve rhythmic (eg running) rather than irregular (eg boxing) movements. Other issues that can impact sensor performance include skin perfusion and wrist tattoos:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
For any workouts that involve flexing your wrists (such as when lifting weights, for example), it may help to move your watch a little higher up your arm, away from the flexing joint.
It may also help to check that the back of your Apple Watch and your wrist are clean:
Cleaning your Apple Watch - Apple Support
If you continue to experience issues with recording your heart rate during workouts, then, for more consistent readings, you may wish to consider pairing an external heart rate monitor / Bluetooth chest strap to your Apple Watch:
Use Bluetooth accessories with your Apple Watch - Apple Support
If you are concerned about a possible hardware defect with your Apple Watch, then I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance:
Apple Watch Service Answer Center - Apple Support