The chart above shows how long each Big Three featured in the Finals has been with their respective teams. As you can see, the Spurs' Big Three has been around a long time.
The last member of San Antonio's Big Three joined the Spurs in 2002. The first member of Miami's Big Three joined the Heat in 2003. Ergo, the entire Spurs Big Three has been in place longer than any members of the Heat's Big Three have been in place.
Since we're so chock full of Big Threes these days, this is a request to start calling the Spurs version "The Long Three." Do I hear a second?