In simpler terms, we can define the rule of the API 3:30:3 as a given time for the developers of websites or applications to attract the most numbers of customers. The first 3 seconds of this rule mean that the customer is dependent on the developer to make their first good impression. Then if the customers tend to stay on your page for 3 seconds, you will have further 30 seconds to offer them what they want in the first place.
Developers should understand what an API does in under 3 seconds, be able to identify the entry point in 30 seconds and start coding with it in under 3 minutes.
Read the entire article here: https://typing.ai/blog/what-is-the-api-3-30-3-rule