On what basis should the merging take place? Your question is rather vague. Also, I assume a, b, …, f are supposed to be strings, that is, ‘a’, ‘b’, …, ‘f’.
>>> x = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> x[3:6] = [''.join(x[3:6])]
>>> x
['a', 'b', 'c', 'def', 'g']
Check out the documentation on sequence types, specifically on mutable sequence types. And perhaps also on string methods.