First let me confess my ignorance about cell phones. I throw myself upon the mercy of the knowledgeable and learned electronic gurus among us. I bought a brand new Blackview BV8000 Pro. It has three SIM card slots. I ordered a SIM card from USmobile and they sent me two sim cards. I put the two nano sized cards in the phone and I still get a message "No SIM card". Should there be a card in the third larger card slot? Is my phone locked (it's supposed to be unlocked)? I don't know what to think. What does the Third SIM card do anyway?