The rule is: the sum of digits on the decimal digits is 10, the product of two digits with the same digits on the unit digits is equal to the product of digits on the decimal digits plus the sum of digits on the unit digits as the thousand and hundred digits of the result, and the digits on the unit digits as the product of the decimal digits and the single digits of the result;

(2) Let one digit be 10a+b and the other digit be 10c+d (and a+c= 10).

Yes (10a+b) (10c+d) =10a+10bc+B2 =100a b+10.

(3)3 12×7 12=222 144,208×808= 168064,

Let one of the three digits be 100a+b, and the other three digits be 100c+b(b is two digits, and a+c= 10).

Then it is (100a+b) (100c+b) =10000ac+100bc+B2 =10000ac+100 (.

So the answer is: (1)1909; 2625; 1649; The sum of digits in decimal places is 10, the product of two digits with the same digits in single places is equal to the product of digits in decimal places plus the sum of digits in single places as the result of thousands and hundreds, and the product of digits in single places as the result of tens and units; (3)222 144; 168064