-Route 3 has snow but does not cause hail in-battle. I'd like to fix this.
You mean it has snow tiles right? A VERY easy fix, A-Map can change the weather, all you need to do is:
1. Load up A-Map
2. Go to Route 3
3. Click Header tab and look at Map Options there should be something for weather.
4. Switch the weather to Steady Snowing or if that doesn't work 3 Snow-Flakes
5. Test it out on your rom.