The templating structure has completely changed in 3.x. There is no way to "migrate" a template from 2.x to a theme for 3.x. You will need to create a new theme -
manual.limesurvey.org/New_Template_System_in_LS3.x
Cheers,
Tony Partner Solutions, code and workarounds presented in these forums are given without any warranty, implied or otherwise.