tdict={'A':{1:['a','b'], 2:['C']}, 'B':{2:['a','b']} }
In [22]: tdict={}


In [23]: f=open('f1')


In [24]: while True:
  ...:   line=f.readline().strip()
  ...:   if not line:
  ...:     break
  ...:   pos1=line.split()[0]
  ...:   pos2=line.split()[1]
  ...:   pos3=line.split()[2]
  ...:   if pos1 not in tdict:
  ...:     tdict[pos1]={}
  ...:     tdict[pos1][pos2]=[pos3]
  ...:   else:
  ...:     if pos2 not in tdict[pos1]:
  ...:       tdict[pos1][pos2]=[pos3]
  ...:     else:
  ...:       tdict[pos1][pos2].append(pos3)
  ...:


In [25]: f.close()


In [26]: tdict
Out[26]: {'A': {'1': ['a', 'b'], '2': ['C']}, 'B': {'2': ['a', 'b']}}

In [27]: tdict['B']['2']
Out[27]: ['a', 'b']