An association has three goals. I was thinking about a survey where a member could select what he/she thinks is important to him/her.

What you think, could selectin a point from triangle be usefull? I.e. clicking in the middle of corner points A and B would mean that goal C is not considered not important at all, and goals A and B are about equally important.

And if so, could that be done in LimeSurvey?

I have had this request in the past and it did not go well. It is impossible to create a triangular clicking area with CSS . You can fake the appearance with border widths but that is not a true triangle. This leaves you with using an image and the math required to determine the relative position of a click gets huge, particularly when accounting for screen-size responsiveness.

If a simple multiple-numeric question is too boring, my suggestion would be to implement a range slider and load 3 inputs with the relative widths of each slider segment. A variation on this question theme - github.com/tpartner/LimeSurvey-Range-Slider-4x-5x