Can you apply for a Thailand Non-B visa extension just days before it expires?

The user is inquiring about the process for extending a Non-B visa, specifically whether the extension can be applied for just 3 days before expiration, as opposed to the commonly referenced rule of applying at least 21 days in advance. A comment clarifies that extensions can actually be applied for on the expiration day itself, indicating that the 21-day rule pertains to obtaining the Non-B visa rather than extending it.