Egg size and baking-- making oatmeal cookies that call for 2 extra lg eggs. Possible to sub 3 lge eggs and get same results?

You're likely to end up with too much egg. I often use 2 eggs and 1 yolk in that case.

2 large eggs will work fine. I wouldn't use 3 unless your eggs are small.

Since my hens produce eggs of wildly varying size, I calculate the total weight of the eggs called for in a recipe (Joy of Cooking has a conversion chart), drag out my handy-dandy kitchen scale, and mix 'n match eggs until I have what I need.

3 large eggs will be too much. An extra large egg weighs 2.25 ounces and large eggs weigh 2 oz. You could whip 3 large eggs and then weigh out 2.50 ounces.

I've just substituted with 2 large in those cases and it's been fine

They only differ in weight by .25 oz, so you would end up with too much egg. It is calculated by the dozen, so when you think about the difference between the smallest extra large egg and the largest large egg, it's hardly a difference at all. I would just go with two large eggs in a cookie recipe. You could also be very exact and whip up an egg, weigh out .5 ounce or 3/4 of a TBS and add it. Personally, I wouldn't.