Here is an ugly iterative solution, in one lambda:

  from itertools import product
  f = lambda n: [
    s for s in
      map(''.join, product("abc", repeat=n))
    if not any(a==b
      for a,b in
        [(s[a:b], s[b:c])
         for a,b,c in
           [(i,i+w,i+2*w)
            for w in range(1,len(s)/2+1)
            for i in range(len(s)-2*w+1)]])]



  >>> f(5)
  ['abaca', 'abacb', 'abcab', 'abcac', 'abcba', 'acaba',
   'acabc', 'acbab', 'acbac', 'acbca', 'babca', 'babcb', 
   'bacab', 'bacba', 'bacbc', 'bcaba', 'bcabc', 'bcacb', 
   'bcbab', 'bcbac', 'cabac', 'cabca', 'cabcb', 'cacba', 
   'cacbc', 'cbabc', 'cbaca', 'cbacb', 'cbcab', 'cbcac']