Hi Laura, welcome to our community 
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Currently, this is not possible unfortunately. The best/easiest way would be to combine these reports in one template. The report container can hold an “infinite” number of items which often helps when combining reports. Another workaround might be to create three preview files (*.LL) and download them from the report server. You can then write a quite simple application that combines these three into one and converts it to PDF then. Helpful posts:
Export und Zusammenführen mehrerer unabhängiger Berichte (German only, however the code is commented in English as well)
We also plan on a List & Label feature that will allow to combine arbitrary reports in version 26. It might also become available in the Report Server sooner or later then.