RoomDao.kt

@Entity
data class Entity(
  @PrimaryKey
  val id: Int,
  val attributes: String,
)

@Entity
data class Entity2(
  @PrimaryKey
  val id: Int,
  val entity1Id: Int,
  val attributes: String,
)

@Entity
data class Entity3(
  @PrimaryKey
  val id: Int,
  val entity1Id: Int,
  val attributes: String,
)

data class EntityMerged(
  @Embedded
  val entity: Entity,
  
  @Embedded
  val entity2: Entity2,
  
  @Embedded
  val entity3: Entity3,
)

@Dao
interface RoomDao {

  @Insert
  suspend fun insertData(entity: Entity)
  
  @Query("select * from entity join entity2 on entity.id = entity2.entity1Id join entity3 on entity.id = entity3.entity1Id")
  suspend fun getAllData(): List<EntityMerged>
}