diff3: add page (#2437)

pages/linux/diff3.md

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+# diff3
+
+> Compare three files line by line.
+
+- Compare files:
+
+`diff3 {{file1}} {{file2}} {{file3}}`
+
+- Show all changes, outlining conflicts:
+
+`diff3 --show-all {{file1}} {{file2}} {{file3}}`