Not sure what you mean with 3k, but with 14-bit lossless and 2.39:1 (as well as SD overclock @240MHz and Global Draw = off) I can go as high as 2560x1072, not higher. Actually, not always continuous rec. even with this setting, so maybe one should consider 2240x938.
2560x1072 is some 2.1x crop to add on top of a native 1.6x APS-C crop... wonder what focal lens one should use in most situations

With 2240x938 you go to 2.4x to be added on top of 1.6x, for a whopping 3.9x crop factor relative to FF

Never understood exactly how these modes are usable in real situations. Maybe for airplane spotting?

Btw I'm unable to make the cropped frame centered on the sensor -- it is slightly on the right or on the left.
Also, fps is locked @30/1.001, seems impossible to lower it.
But not a 70D expert here... pretty sure ArcziPL knows much better.