jmc

We can manually add $3+4+\cdots +12$, or we can use the formula for the sum of an arithmetic series. We multiply the average of the first and last terms $\frac{3+12}{2}$ by the number of terms, $12-3+1=10$. The value of the sum is $\frac{15}{2}\cdot 10=15\cdot 5=75$, so there are $\boxed{75}$ logs in the stack.