It helps when you search the forum first for familiar questions. One hour ago there was almost the same question: https://forum.mendix.com/link/questions/109202
You can create one entity with all attributes that are general for all 3 entities and apply that entity as a generalization to those 3. check the following documentation:
https://docs.mendix.com/apidocs-mxsdk/mxsdk/domain-model-metamodel#3-generalization-relationships