Excel2003

I have data in column A. Data is in groups of three so I can use formulas to
copy into cells in columns B, C, and D; and have been doing so for quite
awhile. But it is time consuming to do this every time. So I need help
automating:

Given:
A1 B1=A1 C1=A2 D1=A3
A2
A3
~
A11998 B11998=A11998 C11998=A11999 D11998=A12000
A11999
A12000

I highlight and copy B1 through D3, then double-tap the small square in the
bottom right corner of the highlighted cells to copy down. The end record
isn't always the same. Sometimes just a few hundred cells, once over 20,000
cells.
When I tried to write a macro to do the same thing, it went all the way down
to A65536.
Please help me to automate this so the copy down process stops when it
reaches the last entry in column A, not the last cell.
--
Kat