How do I know what all options are available as 3rd argument of
It depends on what type of form field you're adding. Full list of default fields is here https://symfony.com/doc/current/forms.html#built-in-field-types and then all available options are listed in the documentation for each form field.
TextType has just a few basic ones https://symfony.com/doc/current/reference/forms/types/text.html.
On the other hand
EntityType has it's own extra options and then inherits all options from its parents https://symfony.com/doc/current/reference/forms/types/entity.html.