Suppose an economy produces three goods (rice, bananas, and strawberries).

Draw its PPF assuming constant opportunity costs, then draw it with increasing opportunity costs.

The PPF with a constant opportunity cost will be a downward sloping straight line.

The PPF with increasing opportunity costs will look like a concave shaped curve bowed outwards.

With three goods, the PPF is a three dimensional figure with three axes, R, B and S.

How do you draw one graph for three products with constant opportunity; then a second graph for the three products with increasing opportunity?