If I remember right E3 had some wierd steps like re-exposure to light and hardening of the emulsion. Not the sort of thing to be done at home. Also labs won't process E3 in E6 or C41 processors. They dont care about your film, They care about what non C41/E6 film will do to their chemicals. This is why some labs will not process film you bulk load yourself. E1,2,3,4,5 and 6 are not just refinements of the same process they are different processes.