You can also use
All instead of Map.
b = Subsets[Range[2, 10], {3}];
In[1]:= b[[All, 1]]
Out[1]= {2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5,
5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 7, 7, 7, 8}
In[2]:= b[[All, 2]]
Out[2]= {3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 6, 6,
6, 6, 7, 7, 7, 8, 8, 9, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 6, 6, 6, 6,
7, 7, 7, 8, 8, 9, 5, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 8, 8, 9, 6, 6,
6, 6, 7, 7, 7, 8, 8, 9, 7, 7, 7, 8, 8, 9, 8, 8, 9, 9}
In[3]:= b[[All, 3]]
Out[3]= {4, 5, 6, 7, 8, 9, 10, 5, 6, 7, 8, 9, 10, 6, 7, 8, 9, 10, 7,
8, 9, 10, 8, 9, 10, 9, 10, 10, 5, 6, 7, 8, 9, 10, 6, 7, 8, 9, 10, 7,
8, 9, 10, 8, 9, 10, 9, 10, 10, 6, 7, 8, 9, 10, 7, 8, 9, 10, 8, 9, 10,
9, 10, 10, 7, 8, 9, 10, 8, 9, 10, 9, 10, 10, 8, 9, 10, 9, 10, 10, 9,
10, 10, 10}
Another (less efficient) way is to use
Transpose.
In[4]:= t = Transpose[b];
In[5]:= t[[1]]
Out[5]= {2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5,
5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 7, 7, 7, 8}
In[6]:= t[[2]]
Out[6]= {3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 6, 6,
6, 6, 7, 7, 7, 8, 8, 9, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 6, 6, 6, 6,
7, 7, 7, 8, 8, 9, 5, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 8, 8, 9, 6, 6,
6, 6, 7, 7, 7, 8, 8, 9, 7, 7, 7, 8, 8, 9, 8, 8, 9, 9}
In[7]:= t[[3]]
Out[7]= {4, 5, 6, 7, 8, 9, 10, 5, 6, 7, 8, 9, 10, 6, 7, 8, 9, 10, 7,
8, 9, 10, 8, 9, 10, 9, 10, 10, 5, 6, 7, 8, 9, 10, 6, 7, 8, 9, 10, 7,
8, 9, 10, 8, 9, 10, 9, 10, 10, 6, 7, 8, 9, 10, 7, 8, 9, 10, 8, 9, 10,
9, 10, 10, 7, 8, 9, 10, 8, 9, 10, 9, 10, 10, 8, 9, 10, 9, 10, 10, 9,
10, 10, 10}