Stacking columns to rows

I have a df with columns names S. no., a, b, c, 1, 2, 3. I want new df with where below each S. no. it will show 1 , 2, 3 and columns will be a, b, c. The values of a, b, c will be repeated thrice for each serial number against 1, 2, 3. I am basically changing 1, 2, 3 columns to rows under each serial number.

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