assuming the obvious way is scaling the x-scale 33.33%.... you can change the current solid properties by selecting the solid, and typing command-shift-y or alt-shift-y to bring up the settings. then, for the width type /3 to divide the current width by 3.
duplicate that solid 2 times and you have your 3 equal solids.
ae does have align tools that are like those in photoshop (not quite as good as illustrator)... so you could set the 'align to' option to 'composition' and the click the align left button, select the next layer and click align center, etc. unlike ai, the distribute functions don't distribute over the composition/artboard (i think that is likely a bug...)
depending on what you are doing, you might be able to use a single layer and a repeater...