Box 1 contains three cards bearing numbers $1,2,3$; box 2 contains five cards bearing numbers $1,2,3$, 4,5 ; and box 3 contains seven cards bearing numbers $1,2,3,4,5,6,7$. A card is drawn from each of the boxes. Let $\mathrm{x}_{\mathrm{i}}$ be the number on the card drawn from the $\mathrm{i}^{\text {th }}$ box, $\mathrm{i}=1,2,3$.

The probability that $\mathbf{x}_1, \mathbf{x}_2, \mathbf{x}_3$ are in an arithmetic progression, is