The 3-year gap after your 12th board exams might be a factor for admission to a B.Sc. (Hons.) Chemistry program, but it doesn't necessarily disqualify you.

A few colleges might have a specific age limit for admission to undergraduate programs. However, this isn't very common for B.Sc. programs. During the admission process (or even in interviews), you might be asked about the reason for the gap. Be prepared to provide a genuine explanation.

Highlight your strong academic performance in your Class 12 exams and any relevant achievements during the gap period. Did you take any courses, internships, or volunteer work related to chemistry during the gap? If so, emphasize these experiences as they demonstrate your continued interest in the field.
If there is a personal reason for the gap, be honest and present your explanation confidently. Focus on how it has helped you mature and prepared you for further studies.

I hope it helps!