The cost of a Lennox 3-ton AC unit can vary depending on the specific model and features included. On average, you can expect to pay anywhere from $3,000 to $8,000 for a Lennox 3-ton AC unit, with some high-end models exceeding this range. Factors that can affect the price include the energy efficiency rating, additional features such as variable speed motors or humidity control, and any installation or labor costs associated with the unit. It is recommended to get quotes from multiple HVAC professionals to get an accurate estimate for your specific needs and location.