Old furnace to new furnace

[fred1949]

My old Geothermal furnace maybe soon getting replaced. The old furnace has 1 large cable going to it. It says 3-3-3 on it. It looks fairly large to me but is there a way to find out what size it actually is. It is about 1/2 inch tall & 1 inch wide. I know the breakers dont always mean anything as far as wire size but it runs to a 60 + 60 duplex breaker.
Thank you,
Fred