There is an undocumented feature of the legend option that allows you to have multiple rows, but last I heard it wasn't implemented for legends placed on the bottom (and, as it is unfinished and undocumented, it may change or disappear at a moments notice). Use the 'legend' option as an object with 'position' and 'maxLines' properties (position accepts the same strings as the string version of the legend option, maxLines takes an integer representing the maximum number of lines for the legend). ie: legend: {position: 'top', maxLines: 3}
As an alternative, you can remove the legend and build your own outside the chart (using the column labels and colors you pass to the chart), which gives you more freedom.