I have a more complex problem with auto building a TOC. (The reason you set them to 1.5 cm is to allow for the extra space taken up by the extra numbers in the Heading 3 level.) Then set all the paragraph indentations to 0 cm and the left tab stops to, say, 1.5 cm. If you want something that looks like this:
HOW TO UPDATE WORD TABLE OF CONTENTS HOW TO
In answer to Praveen’s question in the Comments, here’s how to set the various left alignments.
The Table of Contents tab is the only tab displayed.
HOW TO UPDATE WORD TABLE OF CONTENTS FULL