Draw a square of a medium size. You can make your basis bigger if you want that your drawing will be bigger.
Draw a triangle inside the square in a flat dimension. The lower side is a part of the square.
Erase the square carefully because we don’t need it any more.
Draw a small rectangle in the figure’s upper corner to make it overlap the corner. Pay attention, its width is equal to the width of the triangle at the top.
Erase the rectangle at the top, cutting the corner. Draw such figures at the bottom. They must be slightly inverted.
Carefully erase the rectangles in order to leave the triangle with cut corners.
Draw the second dimension. Draw a straight line down from the left side of cut corner.
Add the same line from the front side of the left corner.
Draw the last horizontal line from the upper side of the right corner. Now we can see that our drawing starts gaining the volume. The lines are not in touch yet.
Draw one more line from the upper left one as if we are drawing a small triangle. Draw one more horizontal part on the right.
All must be connected with the help of the last line, which starts from the first horizontal one.
You must color 3D triangle in a proper way to make it look realistic. You can use one color as it’s shown in the picture, but make the inner details look more darker.
You can also color the 3D figure online.
Choose how you want to paint.
Collage of the stages of the 3D triangle drawing