Stick 3 sticks together to make a triangle. Unless you are using a glue gun, make sure they dry before you carry on.
You can decorate each tree in different ways;
Wind a pipe cleaner around each side of the tree.
Decorate with pompoms
Cut out a square of brown cardboard and stick to one side of the tree as the trunk.
Attach some string to the top of the tree, you can glue it on.