You can use Dimon to help figure out file order. For example,
Dimon -GERT_RECO -dicom_org -quit -infile_pattern "functional/*.IMA"
./GERT_Reco_dicom
to3d will use the order passed to it on the command line. If wildcards are used, then alphabetic order is used, but may not be what you want. You can rename files yourself with your own script, or use Dimon to find the order by looking at the Dicom header for acquisition order.