Median1(c1, c2, c3)

StackVertical(Median1(c1.EF, c2.EF, c3.EF), Median1(c1.OF, c2.OF, c3.OF))

Merge(Median1(c1.EF, c2.EF, c3.EF), Median1(c1.OF, c2.OF, c3.OF))

Note: EF and OF are defined for the sake of brevity as...
function EF(clip c) { return c.SeparateFields().SelectEven() }
function OF(clip c) { return c.SeparateFields().SelectOdd() }