Using a price-per-file model for products that are sold is not recommended. It's intended as a reward above the base price.
The normal operation of the purchase system is as follows:
If someone pays for your project, they gain ownership of the product and will always be able to download the files, regardless of updates or changes to the product's price.
The price-per-file system works as follows:
If someone paid X, they can only download files priced at X or less (And it doesn't matter if he has ownership of the project, but rather the value he paid.).
In your case, if they paid 3 or more at the time of purchase, they will be able to download files priced at 3 or less.
If someone paid you 3, but tomorrow you update and change the file price to 4, that person will NOT be able to download the file.
Individually priced files come with an important distinction regarding ownership: The buyer will only continue to have access to these files if the price they paid satisfies the minimum price you set on the file. This means that if you change the individually priced file to be more expensive, you may lock out access to that purchase tier from existing buyers. We strongly recommend considering how you intend to price your project in the future before using an individually priced file. Use individually priced files for quick bonus content, not to set the price of your content. Use the minimum price on your project to set the price of your product, as this will allow you to change the price at a later time without affecting ownership.