Part of the problem is that different numbers have different widths so tab positions could be an ongoing issue despite your best efforts. For example,
1.1.1 Hello to a Heading 3
2.2.2 Yet another Heading 3
33.2.1 This is another Heading 3 you might see
88.88.88 Another Heading 3 might also have multiple digits

If the space consistency is a real issue for you, then you have two possible options...

You could use a "following number with" space to get a consistent gap between the numbers and the text. Or you could choose to right align the numbers so they flow to the left and the gap to the text is completely consistent.

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Andrew Lockton
Chrysalis Design, Melbourne Australia