MacBook Air briefly wakes from sleep after 3 hours
It may be worth resetting your System Management Controller (SMC) to see if this resolves this behaviour.

To do this on your MacBook Air follow these steps:


Shut down your Mac
Keep the MagSafe adapter (power cable) plugged in.
Press at the same time shiftoptioncontrol (on the left side of the bult-in keyboard) and the power button
Let go
Turn your Mac back on with the power button.


Let me know how you go.

Boot into Safe Mode

Since resetting the SMC didn't resolve this for you, I'd test to see if this still happens when booted into Safe Mode. To do this, follow these steps:


Fully shut down your Mac 
Restart your Mac 
Immediately press the Shift key and keep it down 
Let go of the Shift key when you see the login window (NOTE: If you 
have FileVault enabled you may need to log in twice). 
Take a note of what happens (i.e. Close the lid and check again to see if it still wakes up 3hrs later) 
Exit Safe Mode by restarting your Mac as normal 


Run Apple Diagnostics

Now that we've established Safe Mode makes no difference and resetting the SMC didn't help, it's time to test your Apple hardware. Your model of MacBook Air uses Apple Diagnostics to do this. Follow these steps:


Fully shut down your Mac
Restart your Mac
Immediately press the D key and keep it down until you see the Diagnostics screen appear
Wait for Diagnostics to finish (this typically only takes a few minutes)
Once complete, one of two things will appear on the screen:


a No issues found message
a brief description of any errors found plus further instructions

If the diagnostics test does find errors, take a note of what they are


Note: If pressing and holding the D key at Step 3 doesn't work, start again at Step 1 and, at Step 3 press and hold both the OptionD keys instead. This will try and run diagnostics from the internet instead, so you will need to allow more time for it to complete. 

Regardless, take a note of what happens and let me know how you went.