\[\boxed{\mathbf{3.}\mathbf{107}}\]
\[\textbf{а)}\ 13 \cdot 5 \cdot 2 = 13 \cdot (5 \cdot 2) =\]
\[= 13 \cdot 10 = 130\]
\[\textbf{б)}\ 25 \cdot 34 \cdot 4 = 34 \cdot (25 \cdot 4) =\]
\[= 34 \cdot 100 = 3400\]
\[\textbf{в)}\ 11 \cdot 8 \cdot 50 = 11 \cdot (8 \cdot 50) =\]
\[= 11 \cdot 400 = 4400\]