I am new to Macros but not to Exce.

I am trying to write a macro that will go to cell d3 and automatically
copy that number and paste it into cell a12 and all the way do in column a to where data stops in column b. The data in column b will very every time the
macro is run.

I can do it manually, but when I run the macro it copies the same number
of cell down as used in the original macro. I don't want any data is the cell in column b is empty.

Copy, paste in a12-a54 first time
copy paste next time might be a12-a15 or a12-a101.

Can someone help? Couldn't find answer in any other thread.
Thanks