There are 5,461,512 such combinations.
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64
6 of them.
I am assuming you mean 3-number combinations rather than 3 digit combinations. Otherwise you have to treat 21 as a 2-digit number and equate it to 1-and-2. There are 21C3 combinations = 21*20*19/(3*2*1) = 7980 combinations.
44
There are twelve possible solutions using the rule you stated.