Yes, you can use one A3 earbud on its own for both playback and calls. Both earbuds have the same components (speakers and microphones), which means that you can use either the left or right earbud at your leisure.
To ensure a stable connection, always keep the unused earbud inside the charging case.
It’s always a good idea to charge both earbuds until full after having used them separately to maintain the same battery levels.