If I have an E shaped Core, how can I distribute the gap across the three legs?
To have a gap across the three legs you would need to enter 2 in the parameter N. Gaps. Right now, you can emulate the behavior of the gap in the three legs by selecting number of gaps equal to 2, as it is the number of gaps that the magnetic flux finds in its path, although, as you say, the effect of the fringing flux in the wires would be different.
If you have any other question please let us know.
The Frenetic Team.