I'm off to the store, then! Thanks!
If you don't have a $3 bag of Epsom salt (magnesium sulfate), that should be your first purchase. If you are using RO or bottled water you may need additional calcium, but normally tap or well water will have more than you need especially when added to a quality nutrient.
If you aren't running RO, it's very rare that you need both calcium and magnesium at the same time.
From my reading I remember RM3 likes crazy high sulfur levels and believes it up the number/size of trics. So using sulfates may be your best/cheapest option. Calcium Sulfate, Magnesium Sulfate, ... No need for CalMag.
I'm a big fan of dry nutes/supplements no need to pay for water unless you have to. Especially when they are available in just as good of quality for general uses, aka no Hydro/weed surcharge included.