I agree with Harv, I think the most economical is mixing sand, aggragate and cement yourself in a small electric mixer. That's usually what I do. Well, for smaller jobs anyway.
I use the loader of the tractor to fill my garden cart with sand and pull that around with my lawn tractor. Then I fill the loader bucket with aggragate. I have the cart, bucket, portland cement, water hose and electric mixer all near each other so it's pretty easy to shovel in the proper mix.
I used to use the pre-mixed dry bags, but that stuff gets expensive, and heavy to move around. Also, when storing it it has to stay dry. I can keep the sand and aggragate outside, and just have to keep the few bags of portland cement under cover.
As Harv said, we all have our different ways of doing the job.
I've wondered about the 3 pt. mixers, but I just don't do enough work to justify it.
~Rick