Update cheatsheets

linux/diff3

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+---
+syntax: markdown
+tags: [tldr, linux]
+source: https://github.com/tldr-pages/tldr.git
+---
+# diff3
+
+> Compare three files line by line.
+> More information: <https://www.gnu.org/software/diffutils/manual/html_node/Invoking-diff3.html>.
+
+- Compare files:
+
+`diff3 {{path/to/file1}} {{path/to/file2}} {{path/to/file3}}`
+
+- Show all changes, outlining conflicts:
+
+`diff3 --show-all {{path/to/file1}} {{path/to/file2}} {{path/to/file3}}`