All three sentences are understandable and grammatically fine — they just differ slightly in meaning and tone because of the verb tense and implication of timing. Here’s the breakdown 1. “Where did you get the idea that I would marry you?” → Most natural and commonly used in this context. It sounds hypothetical or sarcastic — perfect if Person B never planned to marry Person A. Meaning: “What made you think I’d ever agree to marry you?” 2. “Where did you get the idea that I was going to marry you?” → Also correct. It implies there was some past situation where Person A might have thought marriage was possible. Meaning: “What made you think, back then, that I was planning to marry you?” 3. “Where did you get the idea that I’m going to marry you?” → Grammatically fine, but sounds a bit odd stylistically because it mixes past (‘did get’) and present (‘I’m going to’). Still, people sometimes use it in conversation for emphasis or sarcasm. Meaning: “What makes you think I’m planning to marry you (now or soon)?” Best choice: 👉 “Where did you get the idea that I would marry you?” — smooth, idiomatic, and emotionally fits the dialogue best.

Yes, all grammatically correct, although a bit strong in many English-speaking cultures.