It depends if you are determining from mass measurements, or a formula calculation.
I assume you took an experment, measured some of the hydrate, heated it, then measured the unhydrated salt.
In that case,
Percenthydrate= 100* (masshydrate-massunhydrated)/masshydrated
Now, to do this to three sig figures might be a problem, as the number of sig digits might be determined by the subtracation, rather than your wishes.
For instance, say
mass hydrated= 10.34 grams
mass unhydrage= 9.51 grams
subtracting, one gets .83 grams, and you cannot get three sig figures from this.
On the other hand, if you are calculating percent water from the formula, then,
[9*molar mass H2O/molar mass Cr(NO3)3*9H2O]*100 = %H2O