Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
If you want to transfer contacts from SIM to Z3, then put the SIM card to the Z3. Open Contacts app. Tap the options button (three dots). Tap Import Contacts and tap SIM. This will import all contacts from SIM card to your Z3.
If you don't have the contacts on the SIM but on the S6 memory, then you can export the contacts to SIM first from S6 and then import to Z3. To do that, open the Contacts app on S6, tap Options tap Settings. Tap Import/Export and tap Export to SIM.
Then import them to Z3.
You can also use Xperia Transfer app to transfer all contents from S6 to Z3. Download Xperia Transfer from Play Store on both devices and follow the onscreen instructions.
See here --> https://play.google.com/store/apps/details?id=com.sonymobile.xperiatransfermobile&hl=en
If you have any questions or would like any additional information, please don't hesitate to ask.