Initially, I had my doubts that this could be done, and then tested it to find it does work. The trick is to give each of Heading 1, 2, and 3 arbitrary names in your source document to be saved as a template. While each heading is selected, make them Place Holder Text from the Format menu > Advanced menu.
The traditional salmon-colored placeholder text will not show unless you single click the header paragraph item.
Insert a new section before your document body containing the paragraph styles, and that is where you will then use the Insert menu > Table of Contents > Document. Now, you are ready to format your Header 2 and 3 indent structure and bullets.
Select the first Heading 1 entry and all Heading 1 entries will be selected. In the side Text panel, go to the Indents section and in the Stops | Alignment | Leader box, cilck the Leader ↕︎ button and choose … . Select the Header 2 entries, repeat this leader dot selection, and then again for the Header 3 paragraphs. You should now have all ToC entities with leader dots.
From the View menu > Show Rulers. Place your cursor in front of the first Header 2 entry. You will note that there will be a bar over a triangle symbol indicating the left margin. With finesse, click and drag only that bar to the right and you will determine your indent by how far you drag it. Select Heading 3 and repeat this process.
Here is the result of my effort:
Now, you are ready to Save as Template into the Template Chooser's My Templates. You might give that saved template a name such as FmtToC.
Create a new document using this saved Template. Click once on each Heading 1 Placeholder text item. It will change to Salmon color and you enter the real, conservative length paragraph name. This will (surprisingly) automatically update that Heading 1 entry in the existing ToC with the new paragraph name. Repeat for Heading 2 and 3 with similar results.
This example is for a default word-processing format document, not a page layout format document.
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Tested: Pages v15.3.1, macOS 26.6.2
