Asking about past relationships isn't weird at all - it's actually pretty normal curiosity, and most people expect it to come up eventually. The key is letting it happen naturally rather than making it feel like an interrogation, so if they mention an ex or you're already talking about something personal, that's your opening to ask follow-up questions about how long they dated or what they learned from it. Three weeks is still early, but you're not out of line; I once asked someone about their dating history around the two-month mark when we were already deep in conversation about family and life goals, and it flowed perfectly because we were already being vulnerable with each other anyway. The timing matters less than the tone - curiosity and genuine interest will land differently than "so tell me about everyone you've dated.