3. Given $f(x)=\left\{\begin{array}{l}(3 a-1) x+4 a, x<1\end{array}\right.$ is a decreasing function on $(-\infty,+\infty)$, then the range of values for $a$ is . $\qquad$
3. $\frac{1}{7} \leqslant a<\frac{1}{3}$ Detailed Explanation: The necessary and sufficient condition for $f(x)$ to be a decreasing function is
$$
\left\{\begin{array} { l } 
{ 3 a - 1 < 0 } \\
{ 0 < a < 1 } \\
{ \operatorname { l i m } _ { x \rightarrow 1 - } f ( x ) \geqslant f ( 1 ) }
\end{array} \Leftrightarrow \left\{\begin{array}{l}
0<a<\frac{1}{3} \\
7 a-1 \geqslant 0
\end{array} \Leftrightarrow \frac{1}{7} \leqslant a<\frac{1}{3} .\right.\right.
$$