If you're just considered sectional headings, then you can use \sections, \subsections and \subsubsections with the following counter redefinitions way:
\renewcommand{\thesection}{\arabic{section}}
\renewcommand{\thesubsection}{\thesection\Alph{subsection}}
\renewcommand{\thesubsubsection}{\thesubsection.\arabic{subsubsection}}
The above changes the numbering scheme representation for sections to \arabic (actually, the default), subsections to \Alph (for theory and examples) and subsubsections to \arabic (also, the default). It also makes sure that the display conforms to your requirements in terms of concatenation.
\sectionso that it produces alternately\sectionAand\sectionB, but I believe that having a specific mark up is better. – egreg Jul 13 '12 at 12:48