The word "trimonthly" is used to describe an event that occurs once every three months. However, there are several synonyms that can be used in place of this word. One alternative is "quarterly," which is a more common term used to describe events that occur every three months. "Tertial" is another synonym that is less commonly used but has the same meaning. "Triquartely" is another option, although it is not widely recognized. In some cases, "every three months" can be used as an alternative to "trimonthly" to make the language more accessible. Overall, there are several options available for those looking for synonyms of "trimonthly".