Since each row will have its own driver, you could easily add an on-off switch to each driver's AC power side, then you could run any combination of 1,2 or 3 rows that works best. For the slight added expense and trouble, you avoid having to take it apart and add a 3rd row if needed. I would definitelly do it that way.
I didn't think about including the on off switches, I never use the ones on my current fixture since they're on timers.
I was hoping, if I end up having enough light with just the two rows, I'd be able to use the leftover LEDs for a small fixture on another tank. So I don't want to install all 3 rows in and end up only needing two. Plus I'm not drilling my heatsink, I'm going to be using the thermal adhesive to mount the LEDs.