A resume that is going to achieve maximum effectiveness does not follow a formula. Every applicant has a different set of skills and qualifications and their resumes should be adjusted accordingly. How to handle short-term employment periods can cause headaches for novice resume writers. The fact is that approximately 40% of employees who left their jobs voluntarily did so within 6 months of their start date.
Before you decide to include or exclude your short-term employment position within the resume, you need to consider all of the factors. First, consider the overall hole that it will leave on your resume if you remove the position. If you had employment immediately before and after a three-month role, then leaving a three or four month gap is not a big deal. But if the removal of the position leaves you with a 18 month gap, you may want to reconsider. Another factor would be where within the timeline the position appears. Was the position within the last year or two, or was it 12 years ago? If exclusion of the position from the resume causes a notable employment gap more than a decade ago, it will be less relevant to the hiring manager than if the gap happened within the last year or two.
Also consider how the position aligns with your employment goals. Was this a 6 month "pay the bills" job that in no way aligns with your career path for the position you are pursuing? Depending upon how the rest of your professional experience on the resume lays out, you may not want this non-relevant role sticking out like a sore thumb near the beginning of your resume. But perhaps it was a temporary job that perfectly aligns with your career that was followed by a long-term role. This is likely a situation where you would include the position, perhaps with a note regarding the temporary status of the position so that your reader doesn't assume you were released for performance issues or that you abandoned the position