How do I drag my Excel formula horizontally across and vertically down, with dynamic reference (every third cell)?

How do I drag my Excel formula horizontally across and vertically down, with dynamic reference (every third cell)?

Forexample,

The data is available as follows (in Row 1, in multiple Columns A,B,C...):

Image1 - Raw Data

I wanted to offset/jump cell every third cell and in following format through formula:

Image 2 - Desired Output