If you are looking for the Joomla 3 Update in the Joomla! Update Component and don't see it ...
...there is a setting that needs to be changed at:
Component >> Joomla! Update >> Options (button top right) >> Update Server >> select: Short Term Support
Don't forget to backup your Joomla 2.5 files and database before Updating.
From the Joomla 3 FAQ which has more information and advice about Updating from Joomla 2.5 to Joomla 3.
