Another way to combine these mixtures is to work with the alcohol and water content directly as fractions of each container's volume, then combine them using the given mixing ratio.
Taking these three solutions in the volume ratio \( 1:2:3 \) and combining the alcohol and water contributions from each container, the resulting mixture works out to an alcohol-to-water ratio of \( 6:7 \).
Therefore, the correct answer is 6:7.