1) Building your own coop may technically save you money, but unless you know what you are doing, it probably isn't worth the time. If I paid myself minimum wage for all the time I've spent finding the materials and making my own coop out of cheap or free scrap wood, it would cost more than ordering one new, lol.
2) You can't really buy chicken food in bulk since it doesn't store well. Organic is crazy expensive in my area. Obviously if you give your ladies access to the compost/large bug-hunting areas, you will have to buy less feed.
3) It would be better to calculate in terms of at least 4 chickens. Three is too small for a flock.
4) I don't think it actually saves you money to buy chicks instead of pullets. First of all, there is no guarantee that your chicks will be girls, even if they are supposed to be. It takes a long time to know what you've really got.
Second, a lot can happen in the five-six months that you are waiting for them to grow old enough to lay eggs. In my experience, that is the most dangerous time in terms of predators (particularly hawks, who are able to fly off with half-grown chickens, but I've never had one get an adult). Meanwhile you have to buy food that whole time, and at the beginning you even have to pay for a chick heater/the energy it takes to run it. Raising chicks is fun but if it's your first time I would recommend starting with 4 or 5 month old pullets. It's worth paying to get a guarantee of gender, health, less waiting time for eggs, lower danger of predators, etc.