The 3DO doesn't have a general video mode like many other systems. There is a VDLP (video display list processor) which drives a DAC which is abstracted by the OS.
ProofVDLEntry(page 64) which shows the number of scanlines being hard coded to 240 (not even a #define). The development docs on the VDL indicate that setting the number of lines in the VDL to 0 means to finish out the field but it appears that's not a hardware feature but a manipulation of the VDL by the
ProofVDLEntryfunction. It tracks the number of lines processed and replaces the 0 with a count of the lines left.