Has anyone told you that you need to decrease the light? That light is only 60W the plant cant handle that and way more. I use 2x150W panels in a 100x100 and I keep them at around 25cm of distance, if I see burned tips I will increase distance, when I cant increase more distance if the plants get too tall I will dimm it a bit, not a lot. So I would turn on the 3 of them and keep them close to the plant, like 20-25cm, because with 60W youre not gonna burn it and it doesnt show any burning on the top tips. If you start seeing burned tips ONLY on the top of the plant then you have too much light but thats not your case. However if the burned tips appear in all of the plant, not only the top, it can mean too much food.