x = {};
n = 3;
k >= 0;
a = 0;
b = 0;
c = 0;
For[k = 1, k <= n, k++,
  br = Input[Row[{"Entre el  lado ", FromCharacterCode[k + 64]}]];
  AppendTo[x, br];]
a = x[[1]];
b = x[[2]];
c = x[[3]];