How do I macro 20,000+ series of data?

The full s/s stretches to nearly 350,000 rows and consists of over 20,000 series of data, between 6 and 30 rows deep, in columns A:J.

In column I of each series, on rows 3, 4 and 5 there are three numbers.
The second number is to be subtracted from the first with the answer being divided by the third.
If possible, the answers should appear in the rows directly beneath these three numbers.

Because there are so many series of data it isn't feasible to manually input the formula and I'm searching for a macro(s) which can do it automatically.

Can anyone please suggest code to help me to this end??
Big thanks.
colwyn.