Trifold futon frames fold in two pieces. It's the mattress that folds into three panels. Because of this, you will need thinner mattresses for a trifold futon frame. Though the method for using a trifold futon is a little counterintuitive, the actual steps are fairly simple. With a little practice, it's a relatively easy process.
Start with the frame in the flat (bed) position. Remove the futon mattress and set down nearby.
Look at the hinge in the mattress, typically halfway between the head and foot of the frame. Find the brace that holds the hinge in position. Rotate it in place until it's no longer locking the frame hinge. Repeat on the other side of the hinge.
Hold the foot of the bed in place as you pull the head half of the frame upward until it's approximately 90 degrees from the plane of the other half. This is much easier with somebody to help you, but can be done alone by stretching your arms to grip both ends at once.
Lock the hinge in the upright position by rotating the braces on both sides of the hinge. In some models, this is the same position as that which locks the frame flat. In others, it's a different position. Consult the owner's manual if in doubt.
Lay the mattress in place on top of the upright frame, draped over the top of the section now forming the back of the couch.
Pull the foot end of the mattress towards the foot end of the frame until the edge of the mattress is flush with the edge of the frame.
Hold the foot of the mattress in place as you fold the mattress, pressing it into the corner of the futon frame. Approximately one third of the mattress with drape over the back and hang behind the futon. This can be done alone, but like folding the frame, is easier with a helper.
Lay the mattress in place on top of the upright frame, draped over the top of the section now forming the back of the couch. The foot end of the mattress should rest in the corner formed by the two planes of the frame.
Keep the foot end of the mattress in the corner as you fold the mattress towards the edge of the frame seat. End with the mattress folded double on the seat of the frame.
Readjust the mattress to align the remaining one third of the mattress with the back of the couch frame.
There are many models of trifold futon frames. These instructions are for one of the more common styles. When in doubt, consult the manual that came with your frame.