From the information I've found it should be 1.75-2.00 grams.
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If it's set too low, it won't track correctly and you won't get all the subtle info out of the bottom/sides of the groove. Too high, (which would need to be over 4 or 5 grams) and you'll get excessive wear on both the cartridge and vinyl.
I have always recommended a tracking force at the upper end of the manufacturer's recommendation range.
I would track the MC3 around 1.9 to 2.0 grams, if it were mine.