I believe it also depends on the type of observation you are adopting. it can be quantitative observation (e.g measures) or qualitative observation (e.g. use of your 5 senses). It depends on which you adopt with the questionnaire to make it qualitative or a mixed method.
Faten - yes it's still multi-methods as David suggests. You are using more than one qualitative data collection method in a single study to, hopefully, address the same topic issue i.e. unstructured/semi-structured questionnaire, interview and observation.
Morse and Niehaus's (2009) interpretation of the four classifications of 'multi-method' design may assist. You just need to identify if your design is simultaneous or sequential:
QUAL + qual—an inductive-simultaneous design
where both components are qualitative; this is a
multi-method design rather than a mixed methods
design;
• QUAL → qual—an inductive-sequential design
where both components are qualitative; multimethod
design;
• QUAN + quan—a deductive-simultaneous design
where both components are quantitative; a multimethod
design;
• QUAN → quan—a deductive-sequential design
where both components are quantitative; a multimethod
Hi Faten, as Dean said, you might need to read more about methodology related to your study. If you are not clear of how to design your research methodology, try to find a study/ article that might help you to replicate the research design.
It seems that your study is qualitative, and that you are willing to employ three sources of collecting data, which in return strengthens the overall trustworthiness of your study findings. The data collected through such tools you mentioned are better analyzed qualitatively, using grounded theory coding, thematic analysis, interpretational analysis, etc.. I would think that such a study is simply qualitative. However, if the questionnaire -you are using- contains many statements/questions that participants respond to, for example, by ticking one response out of 3/4/5-points (Likert scale), then you will employ quantitative analyses, and therefore your study is surely considered as a mixed-method study!