How you accomplish this will depend on two things:

1. Will all the numbers start with 1? If so, just type 1. before the number
field in the "Number format" box.

2. If (as is likely) not, what should cause the numbering to advance to 2?
For example, if your numbering restarts in each chapter (1.1, 1.1.1, 1.2,
etc., in Chapter 1, then 2.1, etc. in Chapter 2), you need to use a
higher-level style as Level 1.

   a. If you actually have a style you're using for the chapter number, you
can apply autonumbering to it and set it as Level 1.

   b. If not, you need to pick a style (your chapter title, for example)
that reliably occurs where you want to restart numbering, make it Level 1,
and apply numbering to it. Format the numbering as Hidden and set "Follow
number with" to "Nothing" instead of a space or tab; the number will not be
displayed or printed, but it will supply the higher-level number for your
first level of numbered paragraphs (Level 2).

To clarify, this is what you would have:

1. Level 1: 1, 2, 3 numbering, Hidden.

2. Level 2: 1, 2, 3 numbering, including Level 1, to give 1.1, 1.2, 1.3,
etc.; restart after Level 1.

3. Level 2: 1, 2, 3 numbering, including Levels 1 and 2, to give 1.1.1,
1.1.2, etc.; restart after Level 2.