This is fun, it's like one of those mystery novels and everybody guesses who the killer may be.
1. Your plant is overfed. Signs of a mild N toxicity are present. Due to the N tox, your plant has produced more leaf tissue, the leaves have become thicker and contain more water.
2. Strangely there are only 3 leaves affected by this mystery burn, neither bottom or top leaves, so it's neither a deficiency, nor a nute burn.
3. Yet one burnt leaf is above another, and on the other side, the other affected one has no other leaves under it. This leads me to think that someone (or even you!) spilled one of the following on the plant and didn't realize or have the guts to fess up: pure nutes, alcohol, chlorine or a pH stabilizer.
4. The thick tissue + the chemical burn will cause damage that most growers will not recognize because, cmon... the odds of these two happening is pretty rare.
Conclusion: not much else you can do about it, maybe take care of the N tox. The burn is there to stay, but won't advance in any way.