The product-specific "Custom field 3" is displayed on the product page in the product specifications.
As the Shopware 6 product specifications have a predefined structure (label, text, formatting), this field is not HTML-capable.
You can edit the label on the product page using the snippet twt.detail.customFields.customField3.label (Texts: Editing and translation).
Note: You can enable/disable the "Product specifications" in the theme configuration.
You can find the "Custom field 3" custom field in the respective product:
Tab "Specifications" ⇒ Block "Custom fields" ⇒ Tab (Set) "Products (ThemeWare® ... Pro)"
Type: Text
Technical name: twt_{theme}_{edition}_custom_field__product__custom_field_3
You can find more information about the "Product specifications" extension here: