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Your usage of square brackets has created the projection on the right-hand side, which is expecting an x argument:
q).my.func:{[x;y;z] (x;y;z)} q)tab col1 col2 ——— a 1 b 2 c 3 / projection q){.my.func[x;y;`field]}'[flip value exec col1,col2 from tab] {.my.func[x;y;`field]}'[((`a;1);(`b;2);(`c;3))] q)type {.my.func[x;y;`field]}'[flip value exec col1,col2 from tab] 104h / right-hand side projection; note .’ as type (‘[x]) will return 106 q)type .'[flip value exec col1,col2 from tab] 104h / using apply-each without square brackets q)type {.my.func[x;y;`field]}.’flip value exec col1,col2 from tab 0h q){.my.func[x;y;`field]}.’flip value exec col1,col2 from tab `a 1 `field `b 2 `field `c 3 `field
Note the usage of apply-each .’
Here is a simplier example of your projection syntax:
/ missing x arg q)@'[1 2] @'[1 2] q)type @'[1 2] 104h / providing x arg q)@'[10+;1 2] 11 12 q)type @'[10+;1 2] 7h
Hope this helps