Here you find some information on how to migrate your templates and paragraphs from Magnolia 2.1.5 to Magnolia 3.0. As you can almost everything within you templates maybe you have to do additional tasks.

First create a package in Magnolia 2.1.5 (use Magnolia Packager migration license) with the files from the web application (jsp, js, gif, jsp, ...) and the respective config entries and install the package in Magnolia 3.0.

The path where to store the template configuration in the config repository has slightly changed:
move your template definitions from path

/modules/templating/Templates

to

/modules/templating/templates

(yes just no capital T).

The path where to store the paragraph configuration in the config repository has slightly changed:
move your paragraph definitions from path

/modules/templating/Paragraphs

to

/modules/templating/paragraphs

(yes just no capital P).

I used the attribute actpage in this tag which is not supported / needed anymore. You have to delete this attribute in your jsp-code. For example change

<cms:out actpage="true" nodeDataName="contentTitle"/>

to

<cms:out nodeDataName="contentTitle"/>