How can I add a new line to a large file every n characters in terminal (one liner sed)?

$ sed 's/.../&\n/g' file.txt

$ grep -oE '.{1,3}' file.txt

ABCDEFGHIJKLMNOPQRSTUVWXYZ

bash shell text sed replace

$ echo abcdefghi | dd cbs=3 conv=unblock 2>/dev/null abcdefghi

bash shell text sed replace

Just with bash:

while read -n 3 chars; do printf "%s\n" "$chars"; done < file.txt > new-file.txt

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