They all are.
Number 1. is probably the anhydrous MgSO4.
Number 2. is probably a hydrated form, MgSO4.n, but the question is which one. n can be 1,2,6,7,or 8. Far and away the most common is the heptahydrate (n = 7), Epsom salts. This is the form brewers usually use.
3. The anhydride is a powder and Epsom salts are certainly crystaline but I don't know for sure.
The difficulty comes when weighing them out. The anhydride will quickly absorb moisture from the air making it difficult to weigh accurately. Uncertainty in the value of n makes it hard to figure out how much to use. The heptahydrate is half water i.e. its molecular weight is twice that of the anhydride and for a given level of sulfate or magnesium you would need to use twice as much (by weight).
It is probably safe to assume that the crystals are heptahydrate (Epsom salts) as I believe that is the most common mineral form.
There are few brewing applications where magensium sulfate is necessary as the malt itself contains more than enough magnesium for enzyme co-factor duty.