Is your power supply at least 2.5A?
Was this a purchase of a new unit? I have read that if you see the old square symbol,
you're trying to boot an outdated Raspbian with boot code that doesn't support the 3B.
Flash a recent image.
And in the future - take heed as you format your little cards for use in the Pi. I believe
32GB and larger is the breakpoint where the card HAS to be formatted one way - DIFFERENT
from cards with less capacity. I did mine wrong, and the little card heated up so much I could
not hold it! But read the instructions (hams, of course, are notorious for NOT reading manuals (grin)),
and all will be well.