OnePlus 3 User Manual page 10

Dual SIM Card Tray
There's no need to lug around two phones or change out your SIM
cards while traveling. Stay connected to two networks, simultaneously!
Instructions:
The SIM tray ejector found in the box is used to remove the SIM tray
from the device. Two nano SIM cards fit into the tray.
10 | What's in the Box Device OxygenOS Safety Information