This rule says you must include any employee in your plan who has worked for you in any three of the last five years, provided they have met your other eligibility requirements. If you use the 3-of-5 rule, any amount of work counts in any three of the five previous calendar years.
This is the most limited eligibility requirement allowed by law. You can choose to use less restrictive participation rules if you prefer. For example, you could let employees participate immediately after they start work or after a shorter period of employment.