demo

$a
   Var1 Var2 Var3
1     a    1    1
4     a    2    1
7     a    1    2
10    a    2    2

$b
   Var1 Var2 Var3
2     b    1    1
5     b    2    1
8     b    1    2
11    b    2    2

$c
   Var1 Var2 Var3
3     c    1    1
6     c    2    1
9     c    1    2
12    c    2    2