Of the 6-digit palindromes composed of 3 distinct digits which can be formed by adding together three 5-digit palindromes:

a) which is the largest?
b) which is closest to a perfect square?
c) what is the largest palindrome you can form by adding some of these 6-digit palindromes together (using at most one copy of each 6-digit palindrome)?
d) same question as (c) but duplicate addends allowed?