The phrase "once every three months" expresses a periodic event that occurs every 90 days. However, there are several synonyms to describe the same frequency of occurrence. These include "quarterly," "every quarter," "four times a year," "tri-annually," and "every 12 weeks." These synonyms provide a variety of ways to convey the same meaning, while also allowing for some variation in tone and style. For instance, "quarterly" might be a more formal or business-like term, while "every 12 weeks" sounds more specific and casual. Nevertheless, all of these synonyms can be used interchangeably to communicate the same idea.