I've used 3 for many years. If it happens to rain, you'll still need to keep one outside all day. It is not unusual to have rain, snow and ice all on the same day here in Southwest Ohio!
Actually a total of 4 is not a bad idea if it happens to be snowing while taking a core for the "Total Snow and Ice on Ground" measurement (if you are looking for super accurate measurements or only want to make one trip outside)
1: Outside on the post, brought inside at observation. (contains the previous 24 hours of precipitation)
2: Inside, taken outside at observation. (collect the next 24 hours of precipitation)
3: Inside, taken outside for "New Snow" core sample (collect previous 24 hours of snow/ice SWE)
4: Inside, taken outside for "Total Snow and Ice on Ground" core sample (collect total snow/ice SWE)