Show a partial list of sections I would like to selectively show a subset of my sections in the main navigation. Is it possible to do something like this?

{% set sections = craft.sections.getSectionById(1,2,3) %}

{% for section in sections}
...
Something like this should work:

{% set sections = [] %}

{% set sections = sections|merge([craft.sections.getSectionById(1)])
{% set sections = sections|merge([craft.sections.getSectionById(2)])
{% set sections = sections|merge([craft.sections.getSectionById(3)])

{% for section in sections %}
    ...
{% endfor %}