I think for a and b the number you are looking for is 199. I knew you needed 1 for the first digit and 9 for the last because the idea is to make the number as small as possible while keeping the digits as large as possible. Basically increasing the hundreds spot hurts you 100 fold up top for each one increase, while only decreasing the ratio marginally. Conversely for the ones digit. For the middle digit to be honest I just tried the different possibilities, nothing fancy.

Not sure about c) sorry its late here and I gotta hit the sack.