3. Given the function
$$
y=(x-m)(x-n)-1(m<n),
$$

and $a, b(a<b)$ are the roots of the equation
$$
(x-m)(x-n)-1=0.
$$

Then the possible order of the real numbers $a, b, m, n$ is (connect with “<”).


3. $a<m<n<b$.

As shown in Figure 5, the function $y=(x-m)(x-n)-1$ is a quadratic function, and its graph is a parabola opening upwards, intersecting the $x$-axis at points $(a, 0)$ and $(b, 0)$.

When $x=m$ or $x=n$, $y=-1<0$. Therefore, $a<m<n<b$.