As rules of thumb go that one is particularly inaccurate. Hope that didn't come from your LBS! Even if you had the largest small chainring, a 39, 3 teeth would still be 8% give or take and smaller chain rings would produce an even larger change in gearing. Let's say your largest cog in the rear was 25. A 1 tooth change is only 4% with larger cogs resulting in an even smaller gearing change.

Conclusion: the "rule" is off by 2:1 even for the most favorable and least likely configuration.