The forbidden built-in
Octave, 32 bytes

@(L)L(x=3:3:end)(diff(L)(x-2)&lt;0)

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L3 = L(3:3:end)  %extract last elements of groups
d= diff(L)       % first difference of the list
y=d(1:3:end)     %extract first elements of each subgroup of the difference
idx = y&lt;0        %check for negative numbers  
result = L3(idx)