So I have a question where the participants should add percentages for 3 sub questions. The sum of all answers must not exceed 100%. Which question type should I use and how to add this last condition. 

Hi, 
the participants have to enter several numerical values.

So you ought to use a "multiple numerical input".

And the sum of these entered values must not exceed a maximum of 100.
This all is built-in.

So please read the manual about this question type and the possible settings
[url] www.limesurvey.org/manual/Question_type_...e_numerical_input/en [/url]

Joffm

I wonder why you didn't answer the questions at the beginning.

I understood what you told me. Thank you for your feedback!

It is all explained in the manual that Joffm linked.