if you want just strings, then one way to do it is to set up and array, and reduce each if condition to a single bit in an integer.
int index = 0;
if (a > 0) index |= 1;
if (b > 0) index |= 2;
if (c > 0) index |= 4;
You can then set up an array of strings that relates to the index value:
string[] selects = new string[] { "Select * FROM none",
"Select * FROM a only",
"Select * FROM b only",
"Select * FROM a and b",
"Select * FROM c only",
"Select * FROM a and c",
"Select * FROM b and c",
"Select * FROM a, b, and c"};
And select your string directly.