Firstly, if you have noticed the filter is damp, wet or damaged in some other way, then it should be replaced immediately. Harmful mold spores or other toxins could be released into the air creating an unsafe environment.
Another example of the need to change the filter more often than 3 months is in homes with large households and pets. This is because the larger the household the more dirt, dust and pet dander tends to accumulate. Also, homes where family members suffer with allergies or that smoke indoors benefit from frequent changing of the filter.
The homes location and surroundings will also have an impact on how often a HVAC filter should be changed. If there is construction going on around you or if you live in area of agriculture, it would be a good idea to replace filters as much as once a month. If there have been wildfires in the area, this can also affect indoor air.
So, as we can see, your homes HVAC filter does a lot to improve indoor air quality so, think about your household and surroundings to determine how frequently it should be changed.