There are three numbers that can form 6 different three-digit numbers. The sum of these six three-digit numbers is equal to 1998. What is the largest three-digit number among them?
有三个数字，能组成6个不相同的三位数，这6个三位数之和等于1998，那么其中最大的那个三位数是多少．
<result>621</result>