Simplify the expression:

To simplify the expression \( -6t^2 + -4t^2 + 2t^2 + -5t^2 + -4t^2 + 3t^2 \), we can combine like terms (the terms with \( t^2 \)).

Let's rewrite the expression without the signs for clarity:

\[ -6t^2 - 4t^2 + 2t^2 - 5t^2 - 4t^2 + 3t^2 \]

Now, let's combine the coefficients of \( t^2 \):

\[ (-6 - 4 + 2 - 5 - 4 + 3)t^2 \]

Now, let's compute the sum of the coefficients step by step:

1. \( -6 - 4 = -10 \)
2. \( -10 + 2 = -8 \)
3. \( -8 - 5 = -13 \)
4. \( -13 - 4 = -17 \)
5. \( -17 + 3 = -14 \)

Thus, the simplified expression is:

\[ -14t^2 \]

So, the final answer is:

\[ \boxed{-14t^2} \]