Assuming that page 1, 2 etc are on level 2 you can do the following:

<ul>
  <xsl:for-each select="$currentPage/ancestor-or-self::*[@level=2 and @isDoc]/*[@isDoc]">
    <li>
      <a href="{umbraco.library:NiceUrl(@id)}>
        <xsl:value-of select="@nodeName" />
      </a>
      <ul>
        <xsl:for-each select="current()/*[@isDoc]">
          <li>
    <li>
      <a href="{umbraco.library:NiceUrl(@id)}>
        <xsl:value-of select="@nodeName" />
      </a>
      <ul>
        <xsl:for-each select="current()/*[@isDoc]">
          <li>
            <a href="{umbraco.library:NiceUrl(@id)}">
              <xsl:value-of select="@nodeName" />
            </a>
          </li>
        </xsl:for-each>
      </ul>
    </li>
  </xsl:for-each> 
</ul>

This is what happens: on whatever child nodes this code gets executed ($currentPage), the parent node with the level of 2 from it will be selected and from there the for-each statement will work on all the direct child nodes. Since you also want to list the grand children another loop is necessary creating another ul in each li. What you definitely need to do is add some if statemets, e.g. if there aren't any grand children then don't even show the ul.

Hope that helps,

Sascha