Well first look at what it costs to buy each compound separately and run it at the dose you want then compare to the price of running that same thing but buying the mix. Usually there is a little price advantage with the mixes but you are stuck with their ratio. So if you wanted to run test higher you would have to buy an extra bottle of test to go with the mixes bottles then measure out how much test you wanted to add in and then take two shots. Me personally I like controlling how much of each I use, that and when I mix three compounds I tend to do different mixes than what they sell.
Don’t get me wrong if you wanted to run the ratio they mixed at then those bottles are great, one shot and done. Nice and easy.
As far as how common are they, just about every UGL does it and everyone claims they were the first lab to make mixes or blends as they call them in the industry.
One thing to keep in mind. If that blend has a compound you have never used before and you end up having issues with it then you are screwed because you can not simply cut out the problem compound and continue with the other two. So I would always advise that you have experience with every compound in the mix before you use them in a blended one shot and done bottle.