Currently, it seems like you can't select more than 2 lines (or any other sketch objects) and apply any constraints apart from "fix" to them. In the below sketch, the three lines are entirely undefined. You should be able to set them all equal, colinear, or horizontal/vertical without having to click on them one two at a time. In the case of 3 lines, it's not that bad, but if you had a complicated sketch with many more lines or other objects and you wanted them all to be equal, it would be much quicker and more efficient to be able to select all of them and hit "equal" instead of having to go through all of them two by two.