Avocado plant leaves going yellow then brown

[CupOfRedTea]

Hi!
Yes, I know, another avocado question
But I'm not sure what the cause of this is, so I'm asking for advice for this particular case. Usually avocado leaves go brown at the tip, but mine start to go yellow, then brown in this pattern, not at the tip. Also it's one of those weird ones with 3 sprouts :S I was going to repot it soon, what should I do, separate them, or leave as is? (Sorry for smudged pics). And should I tie them up to a stick for support or should they be able to support themselves? And is it normal for avocado plants to sit without growth for a few weeks and then grow 20-30 cm in a few days? Or am I doing something wrong?

 

[Fig-Doctor]

Looks like a burn from Chemical fertilizer to me.

 

[CupOfRedTea]

I never used any fertilizers. Could it be from the water?

 

[Fig-Doctor]

It could be. Avocado trees root system is very sensitive compare all other fruit trees. Good Luck!