'Craft requires PHP 7.0 or later'
I'm afraid Craft 3 requires PHP 7. https://docs.craftcms.com/v3/requirements.html

Craft 2 will work with older PHP versions  (5.3 to 7.1.x) so you could fall back to using that. https://docs.craftcms.com/v2/requirements.html

Other than that, I'd say you would need a different host in order to run Craft 3.