What are the 3 grades of steel?

The strength of steel is graded based on it's yield strength- the point that it will have a permenant deformation from a load.

Steel will be graded however an engineer requires it to be in order to meet the design criteria. In order to keeps things simple and economical, common grades are: Mild which is essentially untreated for any design strength; A36 (with a 36,000 psi yield strength); and A51 (with a 51,000 psi yield strength).

Most buildings are designed with A36 in mind and it is by far the most common. Cars, industrial equiment, and special construction may use A51 or higher. Since steel becomes more expensive the more it is treated, any product or assembly requiring anything other than standard yield strength will require an analysis showing the cost savings of using less higher-strength steel versus more lower-strenght steel.

Steel comes in other varieties, like stainless (with its own grading characteristics) or corten (meant to be exposed to the weather).