This works (it's obvious the same will hold for negative integers):

Reduce[a^2 + b^2 == c^2 && 0 < a < 100 && 0 < b < 100 && 0 < c < 100, {a, b, c}, Integers]

This works (it's obvious the same will hold for negative integers):

Reduce[a^2 + b^2 == c^2 && 0 < a < 100 && 0 < b < 100 && 0 < c < 100, {a, b, c}, Integers]

And if you're up for an interesting visualization, here is my take:

ListPlot3D[{a, b, c} /. Solve[a^2 + b^2 == c^2 && 0 < a < 100 && 0 < b < 100 && 
    0 < c < 100, {a, b, c}, Integers], InterpolationOrder -> 0, 
 Mesh -> None, ColorFunction -> "Rainbow", Filling -> Bottom, BoxRatios -> 1]