On 5D3, with the latest ML, external monitors will not give correct framing in higher resolution modes than 1080p. The correct frame preview has low resolution, difficult to focus. As Stef7 mentioned, this is the major caveat of 5D3 ML at the current stage. Other than this preview issue, 5D3 ML is really user friendly, with auto ETTR and 10x zoom in implemented in the 5.8k 1x3 mode.
Frequent changing between the modes freezes the camera on 5D3. To solve this issue, sometimes I take out the batteries, sometimes I copy the new ML files from computer to the CF card. So it is recommended that on 5D3 it is better to stick to one mode and don't change frequently. Update: Unloading the modules of adtg_gui and bench solves this issue. Changing modes now is smooth.
With card spanning and no sd overclocking, I use anamorphic 5.8k 1x3, AR 2.67, 24 fps, 10-bit color depth, 14-bit lossless. It is continuous for a relative complicated scene with trees and leaves at ISO 800 F5.6 or ISO 6400 F16. The reading is about 125 MB/s for the beginning several seconds showing orange color dot, then going green at about 85 MB/s. For super 35mm lenses, I use 3.5k 1x1 centered, AR 2.67, 24 fps, 10-bit color depth, 14-bit lossless. It is continuous for a relative complicated scene at ISO 800 F5.6 or ISO 6400 F16. The reading is about 135 MB/s for the beginning several seconds showing red dot, then going green or orange at about 95 MB/s. The noise at ISO 6400 in 3.5k 1x1 mode is much greater than that in 5.8k 1x3 mode, probably 2 stops difference, not useable to my eyes. ISO 6400 in 5.8k 1x3 mode is acceptable, noise is noticeable.
IQwise, 5.8k 1x3 is little bit better than 3.5k 1x1, in terms of details, not significant though. Both are already very good in full screen mode on a 1920x1080 monitor, if at ISO 100. My experiment showed that 5.8k 1x3 footage is on par with UHD 1x1's in terms of details, if not better. Overall IQ, 5.8k 1x3 is better than UHD 1x1, due to its larger sensor size.
With card spanning and sd overclocking to 160 MHz, I use anamorphic 5.8k 1x3, AR 2.4, 24 fps, 14-bit color depth, 14-bit lossless. It is continuous. I guess that I can also do continuous anamorphic 5.8k 1x3, AR 1.78, 24 fps, 10-bit color depth, 14-bit lossless. For super 35mm lenses, I use 3.5k 1x1 centered, AR 2.4, 24 fps, 10-bit color depth, 14-bit lossless. It is continuous. It seems that the improvement of sd overclocking is not as significant on 5D3 as on 650D at the current stage. I can live with AR 2.67.