On a 3 lead 24h ecg, would it pick up changes from angina like a 12lead?

A 12 lead is better: Depending on the leads and the area affected.It is possible albeit a 12 lead is more definitive. Further testing would rule out ant coronary occlusive disease.

Yes but: It can but the 24 hr monitor is moreso to diagnose any abnormal heart rhythms that might be causing palpitations or dizziness. You are a bit young to be worrying about angina and getting so many tests. I hope everything turns out negative and you can focus on better things.