Hi there!
1. NO, the triangles are not similar
21/12 = 19/11
132 =/ 399
2. YES, the triangles are similar. This is by AngleAngle postulate,
3. NO, the triangles are not similar.
20/12 = 25/15
300 =/ 500
Hope this helps!! :)
If there's anything else that I can help you with, please let me know!