3. Arrange $1,2,3,4,5,6$ randomly in a row, denoted as $a, b, c, d, e, f$, then the probability that $a b c+d e f$ is an odd number is . $\qquad$
Only when $a b c$ and $d e f$ are one odd and one even, $a b c+d e f$ is an odd number, and only when $a, b, c$ are all odd, $a b c$ is an odd number. Let $A=$ “$a b c+\operatorname{def}$ is an odd number”, then $P(A)=\frac{2 \cdot 3!\cdot 3!}{6!}=\frac{1}{10}$.