How to "Synchronously" execute Workflows with Wait Until condition

Hello experts,

I have field Subcategory on an entity. The onChange on that field triggers a Workflow which has a wait condition(waits until the date on another field). The Subcategory field is changed by an email service. The problem is the following:
      User sends Email to change Subcategory in SC1.

User sends Email to change Subcategory in SC2.

User sends Email to change Subcategory in SC3.

The Email service updates once every 5 minutes. If all of those emails were sent in a 5 minutes interval, all of them will be received at once, thus all the changes in the Subcategory field will happen very fast so the 3 Workflows will be fired and each of them will "catch" the same Subcategory, which will be SC3, so the client will get 3 reminders for SC3 when the time is up.

Any ideas on how I can ensure (95%<) that the Workflows will be fired with the Subcategory that fired them ?

Thanks,

Victor