Send reports from all three regions in one email?

EOC only holds the latest stats, so if your reports include historical data (which i would expect almost all good reports should) then it will not serve to aggregate them.

I'd expect the only way to pull this off using Solarwinds tools would be to consolidate those three separate instances into a single instance using additional polling engines and additional web servers.  Outside of that you could do it directly in SQL Server Reporting Services but that would be happening entirely outside the Orion UI.

aLTeReGo​ Do you have any insight into how this may be achieved?

Off hand the only way I can see to achieve this today is with a cross database SQL query. This can be done by specifying multiple instances and database in the FROM clause of your SQL query. E.G.

select
   *
from
  LocalTable,
   [OtherServerName].[OtherDB].[dbo].[OtherTable]