Dissect $3$ equal squares and use the pieces to make $1$ large square.
Bisect $2$ of the squares along their diagonals.
Arrange the right triangles so generated around the outside of the $3$rd square as shown.
Join the $4$ right angles of the $4$ right triangles.
The overlapping bits fit into the gaps.
This dissection can be used as the basis for a tessellation:
This dissection problem was discussed by Abu'l-Wafa Al-Buzjani in a work of his from the $10$th century.