IMHO if you can 5x5 squat three times per week your not lifting enough weight.
Legs once per week is fine, and your always going to hear/ read that 'this' is the best way to train, or do 'that' to grow and explode. Its all relative. Just cuz HIIT is the 'best' cardio fat burner, doesnt mean its right for you. I get along just fine jogging 2-3x week, but I digress(sp?)
But to help with your question, squat once per week, and add in some deads on your back day, space them out as much as possible and you should see some good results.
If however you try 5x5 squats, 3x per week and it works for you the first week then I would highly suggest uping the weight so that you cant do it 3x per week.