iv been playing guitar for 3 years now, completely self taught. I recently had my first ever guitar lesson from a teacher. i played about 3 chords and he basically took everything i knew about playing guitar and rolled it into a ball, spat on it, tossed it out the window, lit it on fire, then drove over it with his car. I was not prepared to change almost every aspect of my play style. he told me i wasnt a real guitarist and that if i did not apply "proper" technique i would never sound good. Apparently iv been holding my pick wrong (3 fingers rather than 2) and im supposed to keep my wrist rather flat and in the same spot right above the pickups.. I am rather hurt and wondering how true his abrasive statements are..
It sounds like you have a bad teacher, honestly. Not because they are entirely wrong, but because that's a terrible way to frame advice to a student. Saying you "aren't a real guitarist" because of these things is just terrible teaching. Instead let me point out some implications of these things and you can decide for yourself.
It is much more common to hold a pick with 2 fingers, the thumb and index, but some people do use 3. My first thought, other than 3 feels awkward to me personally, is that you're giving up a picking finger when "hybrid picking". Most players start off with all down strokes or whatever feels okay and eventually graduate to what is called "alternate picking". Then at some point they'll graduate to using 2 or 3 of the other fingers (that aren't holding the pick) to fingerpick additional parts. That's called hybrid picking and if your middle finger is holding the pick that means you have one less (important) finger to work with.
I can't speak to your wrist without seeing you play. But I will say that where you pick affects your tone. I'd disagree that you should keep it in one place and instead use that tonal variation as a tool at least sparingly. Sure, the default position for the loudest, clearest tone is above the pickup that is currently selected. But you'll notice that if you move higher up towards and even over the neck, you'll get a rounder and more resonant tone. If you move closer to the bridge you'll get a brighter tone and also encounter more string tension. You can use these to your advantage, for timbre, as needed.
More broadly, playing with very bad form can cause you to either get fatigued quickly or cause injury. But usually severe issues like that are corrected naturally as you progress. That is, if playing a certain way hurts your wrist you'll gradually and naturally start playing in a less painful way until you approach something more "proper". If that isn't the case—if you experience pain—then, yes, it's important that you actively look into your form.
Other than that, just look into the implications as I said. Do you think a different technique would enable a different sound or make your playing faster or more efficient? If so, try it out. I've completely reinvented my right hand technique several times at least in my ~20 years of playing.
In general there is no 'proper' or 'right and wrong' in music; rather, there are techniques, and there are the results that you get from those techniques.
If your teacher can explain why a new technique will give you results that you're not going to be able to get from your current technique, then great. If they aren't able to explain this well, that may be a shortcoming of their teaching (although you still may be able to learn things from people who are not able to explain them verbally!)
The one red line is probably when it comes to matters of health and injury; if it a technique is harming you physically I would be happy to agree it's "wrong".
On the other hand, you say...
I was not prepared to change almost every aspect of my play style
to which I would ask: Why not? Sometimes to go in a different direction or learn something new, you do have to begin at the beginning again. But perhaps you wanted to develop something other than your technique, in which case perhaps you could look for a teacher who can give you a more tailored curriculum.
This sounds like the sort of teacher who has the opinion - my way is the only way. If I had the time, I'd go to some lessons and prove to them that it actually isn't. There are 'teachers' around like that. However, I wouldn't waste my time (and money).
Find another teacher, one who has recommendations, and try him instead. There may be a register of tutors available - in UK there's RGT, and MU provides a list of teachers. Other exam boards will probably do the same, depending where in the world you are. Rehearsal studios is somewhere else to ask. And remember - just because someone is a fantastic player does not make them a fantastic teacher!
And - so glad you put "proper" in inverted commas!
From what you say, it sounds like you may have an abusive teacher. To check this, check your own feelings about his teaching style. Was he talking in a kind tone? Did he seem angry?
If you have an abusive teacher, quit immediately and find another one. It will only get worse. When I was learning classical piano, I had an abusive teacher. Things escalated until he finally picked me up and shook me. That ended my piano playing permanently after playing for six years. Now I can only noodle on the piano. I am a pro guitar player so it all worked out but I still miss being able to play fluently on the piano.
At the same time, separate the message from the messenger. Did the content of what he said make sense? After an appropriate period of practicing the techniques mentioned did your playing improve? If so, change your style while you're changing your teacher.
Music is an art form: Part technique, part expression. Some musicians rely on technique, some on expression and some on technique and expression combined. None of the three combinations is the right one. If you have been playing for three years, maybe it a good time to ask yourself what you want to do with you music. You should then find people who whats the same and get inspired from them - including your guitar teacher.