In weight training the term 3x5 is an indication of the number of sets and repetitions to be performed; in this case, you'll do three sets and each set will contain five repetitions. This means you will repeat the exercise five times in a row before stopping to rest or to move on to another exercise; that would be one set. Duplicating this pattern three times means that you performed three sets.
Work to Failure
Typically, a 3x5 variation is performed by experienced bodybuilders only. Most often, the number of repetitions in a set will be to failure, which means that you would not be able to perform one more rep with proper form. If you are a beginner using a level of resistance that causes you to fail in just five reps, it is likely that the weights are too heavy and you may be at risk of sustaining an injury; American College of Sports Medicine guidelines call for most healthy adults to fatigue between eight and 12 repetitions.
Allow for 48 hours rest between training sessions for the same muscle group. Consult with your physician before starting a new weight-training program.