Stack Exchange uses Markdown for its markup language and the Markdown documentation has this to say about list items:
Ordered lists use numbers followed by periods:
1. Bird
2. McHale
3. Parish
It’s important to note that the actual numbers you use to mark the
list have no effect on the HTML output Markdown produces. The HTML
Markdown produces from the above list is:
<ol>
<li>Bird</li>
<li>McHale</li>
<li>Parish</li>
</ol>
So a numbered list is best formed by just doing:
1. Bird
1. McHale
1. Parish
As for having better formatting within the items in your list, like multiple paragraphs, the documentation says:
If list items are separated by blank lines, Markdown will wrap the
items in <p> tags in the HTML output. For example, this input:
* Bird
* Magic
will turn into:
<ul>
<li><p>Bird</p></li>
<li><p>Magic</p></li>
</ul>
List items may consist of multiple paragraphs. Each subsequent
paragraph in a list item must be indented by either 4 spaces or one
tab:
1. This is a list item with two paragraphs. Lorem ipsum dolor
sit amet, consectetuer adipiscing elit. Aliquam hendrerit
mi posuere lectus.
Vestibulum enim wisi, viverra nec, fringilla in, laoreet
vitae, risus. Donec sit amet nisl. Aliquam semper ipsum
sit amet velit.
2. Suspendisse id sem consectetuer libero luctus adipiscing.
To put a blockquote within a list item, the blockquote’s > delimiters
need to be indented.
To put a code block within a list item, the code block needs to be
indented twice — 8 spaces or two tabs.
If you view the source for this answer you'll see how I used those last two statements above through out this answer to format things.
So if you take your example from above, which has the following Markdown format:
1. One
Two
2. List item
Three
1. List item
And use the rules from Daring Fireball, the best you can do is:
1. One
Two
1. List item
Three
1. List item
To get the rendered output:
One
Two
List item
Three
List item
I'm making a bit of a guess on what your expected rendering was, but that's a reasonably advanced example of using lists.