raised garden beds

[sunspot]

I'm going to be putting some raised beds in the vegetable garden. I'd like to use standard 8x8x16" concrete block dry laid, three rows high, with rebar pins. My sister has similar beds two rows high and hasn't had any problem with the rows getting out of plumb. Has anyone gone three rows high? I'm concerned the additional dirt might force the rows out of plumb.

Thanks,