A box $B_1$ contains 1 white ball, 3 red balls and 2 black balls. Another box $B_2$ contains 2 white balls, 3 red balls and 4 black balls. A third box $\mathrm{B}_3$ contains 3 white balls, 4 red balls and 5 black balls.

If 1 ball is drawn from each of the boxes $B_1, B_2$ and $B_3$, the probability that all 3 drawn balls are of the same colour is