Tell us what you are trying to do?
I just need an extra field for some products that customers need to fill.
The field should be a select with 3 options where customers need to chose 1.
Is there any documentation that you are following?
All but no result
I think what you require is a bit more complicated than adding custom fields to products.
If you added a custom field to a product, then that product would have the same field value for all buyers of the product, and I'm guessing what you mean is that when a user buys a product they can add custom fields about the product which get added uniquely to their order.
The common example would be when someone buys flowers, and there is a field where they can add a message for the card.
The Toolset WooCommerce integration doesn't have any way to add such fields as part of the buying process.
You can either implement it yourself, or there are a number of 3rd-party plugins which offer such functionality.