You can send them in with a letter requesting that all applications be processed at the same time. However, be aware that it could result in the hold up of the other two cases if there is an issue with one case (such as getting stuck in name check). I know of a couple who had there interview dates a week apart, but they went to the DO and requested that they be allowed to do it the same day (on the date of the first person's interview). These were elderly folks (80+), so they accomodated them. Not sure what they would do for you.