I'm not familiar with the Orloff, but according to the Henderson Chart, they are fair, not great, egg layers. The Australorp and Rocks are good egg layers. When estimating egg production, I'd use 5 per week from each Australorp and Rock, and 3 per week from the Orloff. You will get more than that in the good weeks, but during molt and extreme weather you will get less.
http://www.ithaca.edu/staff/jhenderson/chooks/chooks.html
As long as you are not trying to breed and maintain a pure breed, I agree a variety in egg color and chicken appearance is nice to have. I don't know how you intend to acquire your chickens, but if it is possible, I'd even suggest getting different colors of Plymouth Rocks. Many people think about the Barred Rock when they say Plymouth Rock, but they also come in Buff, White, Blue, Partridge, and probably a few other colors. It may be difficult to get them, but I can assure you with several of the same breed and color, it can hard to tell the individual birds apart, especially when they are not side by side for comparison.
No matter how safe you think they are, I suggest you get a couple extra. Things do happen and besides, it is a good excuse to get a few more.
I know you did not ask, but when building the coop and run, I'd suggest not using the minimum sizes for planning purposes. They do better with more than the minimum room and when you are first building it is the easiest and best time to provide it. As they get older, after two or three years, egg production will probably drop off and you will have to bring some new chickens in to keep you egg production up. I'd suggest planning for about twice as many as you initially think you are going to get.