Oven Temperature an 3 frozen pot pies
In general ovens are not adjusted for the amount of food being cooked.  It really only matters if you oven is not good at holding temperature, if a very large mass of cold is being heated or if there is enough in the oven to block air movement.  An oven tends to act as a large heat reservoir with a relatively small food item being cooked.  Three small food items will tend to cook in about the same amount of time as one.  You might see a little difference, but it is unlikely to be a huge jump in time needed.  It is different when applied to something like a roast or turkey where a 20 pound item will take much longer than a 10 pound item.

When heating something in a microwave though, this is not true.  A microwave oven focuses the energy and the items in it take that energy without there being a heat reservoir. So, if you attempt to cook 3 items at once rather than 1 you need to provide much more energy, which is done by greatly increasing the time, or simply reverting to doing one at a time.

In the case of conventional oven, I would not really expect a need to increase time by more that a few minutes, if at all, but there cooking times are always approximate in any case.  The true answer to if the item is heated enough is always temperature, not time.