We only support Joomla! 3.4 or later. We strongly advise you to run Joomla! 3.4.7 or later for security reasons.
Support for PHP 5.3 is discontinued. It's end of life since August 2014 and widely considered a security risk, unfit for production sites. Our software requries PHP 5.4 or later and is compatible with PHP 5.4, 5.5, 5.6 and 7.0. We strongly recommend using PHP 5.6 or 7.0.
We'd like to remind you that Joomla! 3.4 does NOT support PHP 7. PHP 7 is only supported by Joomla! 3.5.0 and later versions. Akeeba Backup will work perfectly fine (and very fast!) on a Joomla! 3.5 or later site running on PHP 7.0.