"Shuangfei" and "3P" are often used in online contexts to refer to sexual activities involving multiple people, but they differ in specific reference and social perception. It should be noted that such content may involve vulgar or inappropriate information, and the following distinctions are based solely on objective descriptions in search results, without representing approval or advocacy for related behaviors.

1. Core Definitions and Gender Differences in Participation
| Concept | Definition (based on search result description) | Gender Combination |
|----------|---------------------------------------------------------------------------------------------|----------------------------------------------------------------------------|
| 3P | Refers to sexual activities involving three participants, with no restrictions on gender combinations, which may include various forms such as two men and one woman, two women and one man, etc. | Flexible gender combinations (such as two men and one woman, two women and one man, three men, three women, etc., with no specific limitations). |
| Double Flying | Refers specifically to the sexual act of "two women and one man," which is a specific form of 3P. | Fixed as two women and one man, no other gender combinations. |

2. "Double Fly" in Other Contexts (Non-vulgar Meaning)
It should be noted that "double fly" has a legitimate and neutral meaning in formal contexts, which is completely unrelated to the vulgar interpretation mentioned above:
1. Aviation/Tourism Terminology: Refers to taking a round trip on a plane twice (e.g., "Double Flight Three-Day Tour").
2. Literary imagery: Describing pairs of flying birds, or metaphorically expressing the deep affection between couples (such as 'flying side by side').
3. Four-Nation Army Chess Terminology: Refers to the tactic where the engineers of both sides take turns attacking the enemy's military flag.

3. Social Cognition and Considerations
1. Social Acceptance:
Both "3P" and the vulgar context of "double flying" are considered non-mainstream behaviors, not widely accepted by society, and may carry insulting connotations or provoke moral controversy.
2. Legal and Ethical Risks:
Relevant behaviors may involve health risks (such as the transmission of sexually transmitted diseases) and must respect the will of others to avoid infringing on their rights.
3. Contextual differentiation:
In daily life, one should pay attention to the proper meaning of "double flying" (such as in tourism, literature) to avoid misunderstandings caused by ambiguity.

Summary
In a vulgar context, "double fly" is a subset of "3P," specifically referring to the combination of "two women and one man"; while "3P" covers a broader range of three-person activities. However, it should be emphasized that such topics may involve inappropriate information, and it is advisable to adhere to social moral standards and focus on positive and healthy content. In a neutral context, "double fly" (such as in travel or literature) should be correctly understood according to the context to avoid vulgar interpretations.