To draw a triangle with one square corner (a right triangle), start by drawing a horizontal line for the base. From one end of this line, draw a vertical line upward, which will form the right angle. Finally, connect the top of the vertical line to the other end of the base line with a diagonal line, completing the triangle. Ensure the angle between the base and the vertical line measures 90 degrees for it to have a square corner.
A right-angle triangle is half of a square or rectangle. Draw a square or rectangle and draw a line from one corner to the corner diagonally opposite. You now have two triangles of the same area.
Draw one line vertically - splitting the square in half, and draw one line diagonally - corner to corner.
draw a straight line from one corner to its opposite corner the length of the diagonal is the length of one side times the square root of 2
On a plane surface you cannot. Assuming you mean a right angle when you say a square corner, it is not possible.
There are many different triangles that have an area of three square inches, here's one way to draw one: Draw a rectangle whose area is six square inches (or imagine what one's dimensions would be, e.g. 6x1 or 3x2) and cut it in half diagonally to make a right triangle that has an area of three square inches.