What is the 1-2-3 rule in IFR?
The 1-2-3 rule refers to when an alternate airport is required when planning an IFR flight. If all three parameters are met, then an alternate is not required, but it is always a good idea.
1 hour before to 1 hour after the ETA
2,000 foot ceiling or greater
3 statute miles visibility minimum
How to choose an alternate airport when planning an IFR flight