Three dots are automatically changed to an ellipsis. Why?

Auto replacement of “...” (three periods) to “…” (a single ellipsis character) is due to OS X’s Text Replacement feature. This lets you create snippets for frequently typed text that will be automatically expanded.

To prevent the ellipsis replacement, you can either turn this replacement off in System Preferences ➤ Language & Text ➤ Text:

Replacement_Settings.png

Or turn off all text replacements there, or in Writer:

mac-text-replacement.png

The Text Replacement setting is in the right-click (Control-click) menu, and in Edit ➤ Substitutions