3D palindrome
A palindromic sat number that uses exactly three distinct digits, such as 12,321; the site tracks them inside the Silk Road auction coins.
How a sat qualifies
A sat is a 3D palindrome when its decimal number reads the same both ways (a palindrome) and uses exactly three distinct digits. 12,321, 10,201 and 1,232,321 qualify. 1,234,321 does not: it uses four digits (1, 2, 3 and 4), so it is a palindrome with no further type. 3,443,443 uses two, so it is a 2D palindrome.
The count is of distinct digits, not of length: a sixteen-digit palindrome can still be 3D.
How common
There are far more 3D palindromes than 1D or 2D ones: well over a million sat numbers below the end of the supply qualify. Some are also perfect palinceptions: 123,321,123,321 is the palindrome 123,321 written twice, with three distinct digits.
On this site
The site does not track 3D palindromes on their own, so they have no category of their own. They are tracked inside the coins sold at the first auction of the bitcoin seized from Silk Road, as Silk Road 3D palindromes; that category is listed below.