6. On the front and back of four cards, 0 and 1, 0 and 2, 3 and 4, 5 and 6 are written respectively. By placing any three of them side by side to form a three-digit number, a total of $\qquad$ different three-digit numbers can be obtained.
6.124.

When the hundreds digit is 1 (or 2): $P_{3}^{2} \times 2 \times 2 \times 2=48$.
When the hundreds digit is 3 (or $4,5,6$), and the tens or units digit uses 0 pieces, there are $(05,06,50,60,00)$ 5 repetitions, so
$$
\left(P_{3}^{2} \times 2 \times 2-5\right) \times 4=76 .
$$

Therefore, the total number of different three-digit numbers is $48+76=124$.