This expression can be simplified by factoring out the common factors.

3a^2 - 6ab + 3b^2 - 12c^2

First, factor out the common factor of 3:

3(a^2 - 2ab + b^2 - 4c^2)

Next, factor the quadratic expression inside the parentheses:

3(a - b)^2 - 4c^2

Therefore, the simplified expression is:

3(a - b)^2 - 4c^2