My rule of thumb is that second year seed corn is usually ok. Third year corn likely would have a bit less germination plus less vigor after germination. So it depends on how desperate you are to utilize older seed.

Take 10 corn seed and wrap them in a wet paper towel. Put them in a zip-lock bag. Place it on top of the refrigerator or hot water heater. Check the seed after 3 to 5 days or even wait up 10 days and see how many germinate. If you get 6 or more I would use them. Less than 6 you will need to trash them or plant them thick. I had about 1/2 lb of 2007 yellow hickory king corn seed. I tested this year and got 10 out of 10 to germinate. I gave the seed to a garden web forum member.