I am working a macro that will combine columns. Here's how it needs
to work.

A1 B1 C1 D1
A2 B2 C2 D2
A3 B3 C3 D3

I need to concatenate A1 B1 C1 into D1, then A1 B1 C2 into D2, then A1
B1 C3 into D3, then A1 B2 C1 into D4, then A1 B2 C2 into D5 and so
on. Essentially, I need every combination of columns A through C
concatenated into column D.

Thanks for your help!