I imagine Phoenix would be similar to Tucson in this regard. R3 is typically considered a benefit, because it gives you flexibility. Investors especially like them in Central and University area homes, because they can add additional units to the property. I do not have an exact dollar amount, because it is going to be worth more by the University then it would in a suburb for example.
I would suggest you (or your agent!!) look at several comparable sales in the immediate area in the last 6 months. See if any of them have R2 or R3 lots, vs R1 and if the houses and finishes are the same, that should give you an idea of the value of the R3 lot.
Nicole van Winden &
Two Tucson Natives
Tierra Antigua Realty
Let Natives Work for You!