Side by side videos in shapes
The easiest way would be to use the shapes as track mattes for the video layers. 

Put the Footage A video layer directly beneath the Shape A layer and in the Modes column choose Alpha Matte "Shape A" from the Track Matte drop-down. Repeat for Footage B and Shape B and Footage C and Shape C



What track mattes do is use the alpha (transparency) or luma (brightness) of a layer to set the transparency of the layer directly below it. The Alpha Inverted option reverses wht is visible or invisible.

There's always at least two ways to do anything in After Effects, so I should just mention that other ways to do the same thing include copying the shape paths and pasting as masks on the video layers, or using the silhouette alpha mode on the shape layers and pre-comping. All methods have advantages and disadvantages. Read the page here for more.