One to Many Lists Merge

I have two lists:

list1 = {{1, A, AA}, {2, B, BB}, {3, C, CC}, {4, D, DD}, {6, F, FF}};

list2 = {{1, a, aa}, {1, b, bb}, {1, c, cc}, {2, d, dd}, {2, e, ee}, {3, f, ff}, 
         {3, g, gg}, {3, h, hh}, {4, i, ii}, {4, j, jj}, {4, k, kk}, {4, l, ll}, 
         {4, m, mm}, {5, n, nn}, {5, o, oo}, {7, p, pp}};

I want to merge these two lists. While merging list1 to list2, elements of list1 should be repeated or missing as shown below:

result = {{1, a, aa, A, AA}, {1, b, bb, A, AA}, {1, c, cc, A, AA}, {2, d, dd, B, BB}, 
          {2, e, ee, B, BB}, {3, f, ff, C, CC}, {3, g, gg, C, CC}, {3, h, hh, C, CC}, 
          {4, i, ii, D, DD}, {4, j, jj, D, DD}, {4, k, kk, D, DD}, {4, l, ll, D, DD}, 
          {4, m, mm, D, DD}, {5, n, nn, Missing[], Missing[]}, 
          {5, o, oo, Missing[], Missing[]}, {6, Missing[], Missing[], F, FF}, 
          {7, p, pp, Missing[], Missing[]}};

Any help is greatly appreciated. Similarly, if we can merge above two lists plus the following list in a similar manner in one step, please help me.

list3 = {{2, .2, 20}, {3, .3, 30}, {4, .4, 40}, {5,.5, 50}, {6, .6, 60}};

Thank you all in advance.

Answer

Approach using v10 functions:

CustomMerge1[lists__] := Module[{assocList, keys, vals, res},
   assocList = GroupBy[#, First -> Rest] & /@ {lists};
   keys = Sort@DeleteDuplicates[Flatten[Keys /@ assocList]];
   res = Table[
     vals = #[k] /. Missing[___] -> {{Missing[], Missing[]}} & /@ 
       assocList;

     Flatten /@ Tuples[{{k}, Sequence @@ vals}]
     , {k, keys}];
   Flatten[res, 1]
   ];
CustomMerge1[list1, list2, list3]
(*{{1, A, AA, a, aa, Missing[], Missing[]}, {1, A, AA, b, bb, Missing[],
   Missing[]}, {1, A, AA, c, cc, Missing[], Missing[]}, {2, B, BB, d, 
  dd, 0.2, 20}, {2, B, BB, e, ee, 0.2, 20}, {3, C, CC, f, ff, 0.3, 
  30}, {3, C, CC, g, gg, 0.3, 30}, {3, C, CC, h, hh, 0.3, 30}, {4, D, 
  DD, i, ii, 0.4, 40}, {4, D, DD, j, jj, 0.4, 40}, {4, D, DD, 4, kk, 

  0.4, 40}, {4, D, DD, l, ll, 0.4, 40}, {4, D, DD, m, mm, 0.4, 
  40}, {5, Missing[], Missing[], n, nn, 0.5, 50}, {5, Missing[], 
  Missing[], o, oo, 0.5, 50}, {6, F, FF, Missing[], Missing[], 0.6, 
  60}, {7, Missing[], Missing[], p, pp, Missing[], Missing[]}}*)

As you see this function work with any number of input lists