Daniella,
You should be fine with a KH of 3. I have a KH of 2 and my PH doesn't swing too much after a few days. I do however do daily 100% waterchanges so that keeps my PH stable. I've read that nitrification in itself can begin to lower KH over time, so you could possibly begin to have PH instability over time. Why don't you try it and see how it goes, testing the KH/PH twice daily for 3 or 4 days without a water change.
Eddie