What's crackin the leaf?

Ok I've already lifted lamp a little bit, hope this will help. Thanks

That is magnesium deficiency and if all 3 plants are not together in same dirt or water system, then ph is not always ok. You should measure your ph by the run off so you know what your soil or substrate ph is if in a potter. Plus every plant is different, even same seeds in a pack. Closer to the light, and warmer it is the faster the plant will go through water and nutrients. It has a limit as well. Good luck and hopefully I helped. Happy growing 🍻

Lightburn typically will happen on leaves closest to the light. If its on the lower leaves only id rule that out. Is there a fan blowing hard on that spot? Could be wind burn. Beyond that it could be magnesium def so a little calmag would help.

Looking like a lightburn to me,,,, your lights may be to close on that plant