The Beck Depression Inventory (BDI-II), Hopkins Symptom Checklist (HSCL-25), and Posttraumatic Diagnostic Scale (PDS) are three widely applied clinical instruments for assessing depression, anxiety, and posttraumatic stress symptoms, respectively. Use of online-based psychological help and assessment is rapidly growing which necessitates the need for the validation of online assessment. To address these needs, data from 1,544 Arabic mother tongue treatment-seeking participants, who filled in the Arabic versions of these instruments online, was analyzed in two steps. In the first step, exploratory structural equation modeling (ESEM) was used to scrutinize factorial validity and eliminate items. In the second step, we examined the interrelationships between the latent factors (dimensions) using confirmatory factor analysis (CFA) of multitrait-multimethod (MTMM) data. Results show an acceptable to good fit of the hypothesized model, providing some first insights into the factorial and construct validity of the Arabic versions of BDI-II, HSCL-25, and PDS under consideration of cultural-specific aspects. Present evidence speaks for construct validity of the three instruments and the reliability and usefulness of online assessment.