I moved this from the More Sony Phones forum to the Z3 forum, since it's pretty close.
The first thing to do is to see if any particular app or process is the primary battery drain, as well as to see if poor cell signal is contributing. You've probably seen this from me before, but: Charge up to 100%, then let it drain to 20-30% with normal usage, without charging in between. Then show us:
1. The main Settings>Battery screen.2. The fullscreen battery graph screen (which shows Awake time and Cell Signal Strength).
3. The Screen category (to see total Screen On time).
4. The Cell Standby category, if present (to see Time Without Signal).
You might have to install a 3rd party battery monitor, like GSam Battery Monitor, to get the Cell Signal Strength graph.