Using perfect squares:

This is super tricky as some of the smaller perfect squares that are 3 digits repeat digits (like 121, 144, and 225). If we try 169 (whose square root is 13), then many of the two digit perfect squares cannot be used like 81, 64, 49, and 36).

For example, sqrt (169) – sqrt(25) – sqrt(4) would give 6.

If we bump up to 256 (whose square root is 16), then we can use 81 for the two digit and 9 for the single digit making it 16 – 9 – 3 = 4.

Using non-perfect squares:

Sqrt(145) – sqrt(92) – sqrt(6) = 0.00044178938368

Credit and thanks to openmiddle (if you use these in the classroom, try to modify the instructions to make them less Google-able)