Accumulator #3 is probably one of the more confusing Accumulators. That's how I find it anyway. Probably because it's interrelated with so many other things.
I prefer to view it as the angle formed between Clubshaft and Left Arm when the Wrist is Level as drewitgolf said earlier. There can only be an angle formed if you put the grip under your heel pad. If it goes in the thumb pad (e.g. in putting), then there won't be an angle when your Wrist is Level.
The confusing bit is that if your Left wrist is Cocked (i.e. Accumulator #2), that also counts as accumulator #3 even though you may initially have zero Clubshaft/Left Arm angle when your Left Wrist was Level.
In case you were wondering what "maintaining its radial alignment" means, it's basically about keeping a Flat Left Wrist, so the Clubshaft and Left Arm forms a straight line when you're looking perpendicular at it (but not if you're looking sideways because of the Acc #3 angle). So in that sense, it's never "out-of-line", but in-line.
All other things being equal, more Accumulator #3 equals greater Clubhead travel distance with the same amount of Roll with your Left Wrist. This equates to more Clubhead speed. But all other things are not equal because the more Accumulator #3 you have, the sooner you have to start Uncocking your Left Wrist (releasing Accumulator #2) which prevents the Trigger Delay needed in a Snap Release.
For those still incubating, you must make the link between the following:
Rhythm
Hinge Action
Flat Left Wrist
Accumulator #3
Clubhead Travel
Left Wrist Roll/Motion
Lag Loading
The above list may confuse you even more (I hope not!), or it may help you. Due to time constraints at the moment, I cannot go into detail about their interrelatedness.
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