Are there any editors that can edit multi-gigabyte text files, perhaps by only loading small portions into memory at once? It doesn't seem like Vim can handle it =(
If you are on *nix (and assuming you have to modify only parts of file (and rarely)), you may split the files (using the split command), edit them individually (using awk, sed, or something similar) and concatenate them after you are done.
cat file2 file3 >> file1