What you are thinking of attempting is very difficult, and even if you use the same brand of paint each time, you may get slightly different results—in fact, even out of the same bottle, if you don't mix it completely every time.
That said, it can be done. But even when paint is manufactured, supposedly with very uniform raw materials, the shader (the technician who matches the paint to a standard) starts with an approximation, then adds different pigments to obtain an acceptable match. Note that I did not say a perfect match. The red, yellow, and blue you are thinking of using are not probably not pure tints, such as the shader uses, but rather contain small amounts of other pigments, not to mention flatting an hide agents.
Why not order the colors you need online? Because my "local" hobby shops don't carry the brand I prefer, I buy almost all my paint that way.
Good luck!