It sounds like you need to confirm your Google account is set up on both your A3 phone and your Tab S2. And it's kind of a semantics issue but it's not a matter of your A3 and S2 are syncing with each other, technically both your mobile devices are syncing with your online Google account. Your Google account is the central storage for your calendar data.
On both, check your Settings >> Accounts >> Google. Your gmail address should be listed in both and if you tap on it the 'Sync Calendar' option should have a green check mark, and tap the 'Sync now' button at the bottom just so it does an initial syncing refresh.
