easily just add a text file named setup.ini with the following in it ( place next to setup.exe this has been mentioned 100's times already)
Code:
[ExtractSettings]
FreeArcFile1=Data1.cab;DestDir:{app};Disk:1;
FreeArcFile2=Data2.cab;DestDir:{app};Disk:2;
FreeArcFile3=Data3.cab;DestDir:{app};Disk:3;