Mixing memory in triple channel board

We recently bought an i7 X58 server with 6 memory slots. Since it was cheaper, we opted to get the min memory configuration with the machine (3x1Gb) and then buy 3x4Gb separately. It seems a shame to waste the 3Gb, so I'm wondering how to "mix" the memory without sending everything into single channel mode. The information I've read is rather confusing, and often contradictory. Would appreciate some advice, thanks !

If it's set up for triple channel, then the slots must be related in 3s. So stick the 3x4GB in the first matched slots and the 3x1GB in the second matched slots. Or the other way around, as long as the matched sticks stay in the same group as each other. Usually the slots are grouped by colour.

If the slots are like

a A b B c C

Then stick 4GB sticks in a,b,c
Stick 1GB sticks in A,B,C

a,b,c are a different colour to A,B,C

Like this one : http://www.scan.co.uk/Product.aspx?W...source=froogle