Gfretwell gave you the correct answer.
When ordering or specifying concrete today, it is done by requesting a PSI strength depending on how the concrete is to be used. 2500# for residential footings, minimum; most commercial work is speced at 3500#; tilt up and prestress run 4500-6000#. Each excess gallon on water per yard above the design water/cement ratio reduces the strength by 500#, so you will often see references to not using too much water.
The very old method of ordering, mixing, or thinking about concrete as xxx number of sacks per yard is not used anymore, but most concrete people (at least we old ones) do understand the terminology.
It kinda ends up knowing what you are doing with the concrete, the stresses to which it will be exposed, your expectations for the finish product, etc. 3 sack will set up, get hard, and will crack. 6 sack will set up, get hard, and will crack. The six sack can take a great deal more load before crushing.
Your call. There is an excellent explanation with facts and figures here:
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