What are three ways to build a pillow fort?

You could use chairs or sofas in the room as stands, draping large sheets across them to form a canopy and walls. Alternatively, you could stick the corners of the sheets to a corner of the room so the walls are pre-built in, allowing for as tall a canopy as you want, but having a less cozy feel overall. As another option, you could use other large items besides blankets and sheets such as large umbrellas, decorating the space with lots of pillows and stuffed animals to cover any item that isn't already soft or fluffy.