Hi

there could be many reasons for such an error .what basically happens the control file store the structural information of the database like physical loaction number and name of datafiles etc.now during the mount stage the instance opens the control file reads its contents and in the next stage it tries to open the database now in your case oracle had the information that there were 3 datafiles and when it tried to open the database it found only one (my guess is it first checked for thsi file ) it aborted without opening the database.you could do one thing.if the datafiles dont belong to system tablespace

say startup mount
alter database datafile name of datfile offline
now say
alter database open........

i hope i have answered your question to some extent atleast.now if you ask me why it did not check for all three files my best guess is when oracle found the first datafile it was lookin for was missin it just refused to open.but this is a guess only i am waitin for gurus to chip in

regards
hrishy