Hi @beunite,
The "extends automatically every three months" depends on the particular software used to obtain the certificate (some do and some don't), and in the case of certificates obtained by the hosting provider on behalf of the user, also depends on the hosting provider's policies and practices. So I wouldn't encourage people in general to rely on this kind of statement without further investigation. (In particular, it's not fully "automatic" at Let's Encrypt's end; someone always has to do something on the other end to make it automatic.)
Fortunately in your case we have received a report in December 2015
https://community.letsencrypt.org/t/web-hosting-who-support-lets-encrypt/6920/17
that novatrend.ch was one of the first providers to offer fully automated support for renewal, handled by the hosting company directly. Thus, on the basis of this report and what novatrend.ch support told you, I think it's most likely that the certificate will be renewed automatically on your behalf by your hosting company and that, in your setup, you don't have to take additional steps.
I guess you can also ask the hosting provider how soon before its expiration the certificate will be renewed, in order to confirm that this really takes place at the expected time. Hopefully their practice isn't to renew it at the last possible moment (just in case something goes wrong technically).