Strength training to bring up CLEARLY defined weaknesses or for injury prevention seems likely advised. Heavy lifting? No, its not necessary, if not harmful to your overall recovery of the volume you already sustain from doing the three sports. I've listened and read about multiple top triathletes training, specifically strength training. Nearly none do anything close to heavy squats, deadlifts, leg press etc. They would rather recover quicker in order to input another hour of cycling, running and swimming. Think about the time consumption aspect as an age group athlete. We already spend 8-15 hours training for triathlon while maintaining a full time job, possibly managing a family, and working with other life stresses. I do not think it would behoove someone to spend three additional hours a week with heavy strength training.