If you want to change the value of the variable a inside another section, you shouldn't name the section:

Here is a simple sample:
this is wrong:
{let a=1}
{$a}<br>
{section name=Loop1 loop=3}

{section name=Loop2 loop=3}

{section name=Loop3 loop=3}
{set a=3}
{$a}<br>
{/section}

{set a=2}
{$a}<br>
{/section}


{$a}<br>
{/section}
{/let}
----------------------------------
Instead use the following, without naming the sections:

{let a=1}
{$a}<br>
{section loop=3}

{section loop=3}

{section loop=3}
{set a=3}
{$a}<br>
{/section}

{set a=2}
{$a}<br>
{/section}


{$a}<br>
{/section}
{/let}