Add subsite into the master page Top link bar.

 Set the MaximumDynamicDisplayLevels="3".
Example:
<SharePoint:AspMenu
      ID="TopNavigationMenuV4"
      Runat="server"
      EnableViewState="false"
      DataSourceID="topSiteMap"
      AccessKey="<%$Resources:wss,navigation_accesskey%>"
      UseSimpleRendering="true"
      UseSeparateCss="false"
      Orientation="Horizontal"
      StaticDisplayLevels="2"
      MaximumDynamicDisplayLevels="3"
      SkipLinkText=""
      CssClass="s4-tn"/>