how to write this MapReduce

[Anty]

Hi:
all
I have a such use case:i have three files,each file is key-value pairs,
file1:                         file2:                         file3:
key1-value1A           key1-value1B           key1-value1C
key2-value2A           key2-value2B           key2-value2C
key3-value3A           kye3-value3B           kye3-value3C
   .....                                  ......
.....
now ,i want to write a MR job to generate a file,
file4:
key1-(value1A,value1B,value1C)
key2-(value2A,value2B,value2C)
key3-(value3A,value3B,value3C)
..........
Any suggestion will be appreciated.
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Best Regards
Anty Rao