To put it simply, the aspirin is needed because it makes the ephedrine and caffeine more effective.
Aspirin inhibits an enzyme (cyclooxygenase) which produces primarily two ligands: prostaglandins and thromboxanes. Prostaglandins inhibit lipolysis in cells. So if you lower the production of prostaglandins, there is the potential for a higher occurance of lipolysis. Which equals a higher use of fat for "fuel" in the cells.
The one time I ran ECA the 3x daily protocol worked for me. I also have a high metabolism, so stimulants don't really affect my quality of sleep. Depends on your sensitivity.
I dosed ECA at 25mg/200mg/325mg respectively. I've also broken up the ephedrine tablets in two, and taken 12.5mg about every 2-3 hours. I've heard this is more effective... who knows. Ephedrine has a pretty short half life anyways.