what three substances are usually electrolytes?

Answer 1

You mean name three? There are literally hundreds.
Solutions of NaOH, HCl, NaCl, Na2CO3, Na2SO4 etc etc and all of these are STRONG electrolytes (not weak electrolytes).

Answer 2

Sodium chloride- (table salt)

Hydrocholoric acid- (used in household cleaning supplies)

Potassium nitrate- (used in fireworks)

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