It's possible you might be thinking of:
* A sentence with a specific grammatical structure: For example, a sentence with a specific verb tense or subject-verb agreement pattern.
* A sentence within a particular linguistic theory or model: Certain grammatical frameworks might have their own internal classifications.
To better understand what you're looking for, please provide more context:
* Where did you encounter the term "3a sentence"?
* What is the specific context or topic related to this term?
With more information, I can help you understand what you're looking for.