In many countries, there has been a development towards the inclusion of students with special educational needs in regular education. Over the past decade, this has resulted in an increased interest in attitudes towards this educational change of those directly involved. This current study aims at the development, psychometric evaluation, and validation of a questionnaire to measure attitudes of teachers, parents, and students. The questionnaire is based on the three-component theory, reflecting a cognitive, an affective, and a behavioral component. Based on data of a pilot study and a main study, we evaluated the psychometric properties and separability of the three components per questionnaire using a Mokken scale analysis. Removing various dysfunctional items resulted in questionnaires with appropriate psychometric properties and high reliability. Results of the analysis for the separability of the attitude components revealed no distinction could be made between the components in the teachers' and parents' questionnaire. In the students' questionnaire, the items belonging to the cognitive and affective/behavioral components were distinguished as two scales. (Contains 6 tables.)