Attach a short piece of invisible thread 3 to 4 inches long to a small piece of tape. When no one is looking, attach the thread to a poker chip. You can use a large coin or other flat object if no poker chips are on hand.
Set the chip back on the table with the tape side down. The string should be invisible to your audience, and you should only notice it because you know what to look for.
Announce that you are going to perform a mind-reading trick. Ask the audience to inspect the three identical cups, and then place one cup over the poker chip.
Tell the audience that they may mix the cups up as much as they like, but make sure that they concentrate on which cup is covering the poker chip. Turn your back and allow them to shuffle the cups.
Turn back around and pretend to read their minds to find out which cup the chip is under. Look for the invisible thread and reveal the chip. Allow the audience to inspect the cups to make sure that they're all identical while you remove the thread and tape.