siri cannot hear me on my apple watch series 3

Is the problem only Siri? Has it ever worked? Try a spoken reply in iMessage as a simple test, or try making a voice call from the watch.

If other voice apps work ok I suggest unpairing and re-pairing to clean up the software.

If none of the voice apps work, check you didn’t get the water lock activated or water in the microphone. Check the mic isn’t obstructed (third party protective case?). If it still doesn’t work, your watch should still be in warranty so Contact Apple Support and arrange to get the watch inspected.