Def starts as discoloration. Noticed in the veins, on the leaf, etc. the entire plant should be about the same color so when you see random veins that are bright green on one leaf or branch it usually symbolizes a def, now on top of the fact that you have 3 plants 1 on pot, simple plant growing says they're going to fight for nutes, and i dont know how much you've added or whats in your soil by default, however just think about having to give ONE plant a proper feeding for it to grow correctly, now account for the fact you have 3 in one pot constantly fighting one another. when you hit flow, thats when the plant is really going to show you your def's.
I appreciate the effort, but please forgive me if I ignore all of your advice:
1) The random yellow birthmark is genetic, not nute deficiency (that's not even what any nute deficiency looks likes). Furthermore, even it looked like that, localized discoloration is NOT how nute deficiency manifests itself (when you have a protein deficiency, you don't lose one muscle at a time). Most deficiencies (if not all) affect the entire plant, or one symmetrical section (top nodes, stem, older leaves, etc) not one branch at a time.
2) I understand why 3 in 1 pot is inadvisable; but to suggest that they will be deficient as a result is logically flawed. In order for them to suffer deficiency, they would need to grow to an unsupportable size, which will not happen under these conditions. The only issue I will face with this 3-in-1 method is poor size, but NOT poor health (although that is possible for other reasons). The general concept is now of 1 bushy plant, not 3 separate ones.
3) Deficiency doesn't always start as discoloration - there are many, many different deficiencies to understand in this game; and if your understanding is "they looks sad, give them all tha nutes!", then I really feel sorry for your plants.
Your understanding of "simple plant growing" is quite shaky, my friend. I appreciate and accept advice, and will respond pleasantly when I should - but bad advise that is going to hurt my babies will be remorselessly rejected.