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Each size can be popped out of that. The Nano SIM is slightly bigger than the actual contacts for the pins inside of the device. You should be able to pop it out without too much of an effort.
What the whole 3-in-1 SIM means, is that depending on the size you need, you can pop it out. The largest size SIM has two smaller sizes. As mentioned, the Nano SIM is literally just the gold area with a slight bit of white around it. If you flip the SIM over so the gold contacts are face down. Do you see a groove around basically the size of the gold area on the other side? If so, that will pop out.
SIM trays can vary based on the phone. Does your SIM slide in after taking the back off of the phone or does it use a SIM tray that slides out of the side? Some phones also take a MicroSD card. Make sure you are putting the right size SIM in the right place.