What is the syntax for calling multiple SSIS packages (Package1, Package2, Package3) using dtexec in a bat file?
dtexec /f "C:\Package1.dtsx"
To start the packages in sequence, I guess that you just can create a batch file where you start them.
dtexec /f "C:\Package1.dtsx"
dtexec /f "C:\Package2.dtsx"
dtexec /f "C:\Package3.dtsx"
To start them in parallel you can workaround the limitation of dtexec by calling it in separate command. I found this post that explains that: http://www.rafael-salas.com/2010/07/ssis-how-to-run-set-of-packages.html
You can create a batch of packages where you cmd.exe for each of them.