You have two output connectors, you can have only two monitors. You would need another card to support a third display device. I do not have CS5 so I cannot answer about CS5, but I would assume it will let you direct the output like CS4 did..
Why wouldn't the "y" adapter work?
If not...can you recommend a second "decent priced" video card for HD monitoring? Simply plug it in and tell Premiere to out put to that card.
My main card is the GTX285.
As far as I know the only thing a "Y" adapter could do would be to output the same signal to both monitors. Unless your "Y" adaptor has some smart circuitry that I am not aware of.
Just put any old card that you have in your system and see if it works. My only concern would be dirver confusion.
Why dont you just use the 2nd monitor as the Playback preview monitor via the playback settings?
I need two monitors for my desktop editing...
I vary my "previews in a few ways.
I have 2x PC monitors running the NLE. (plus I have 2 additional external HD monitors online also)
I preview using the tilde option
I preview to the second monitor
I preview to my externals via f/w and a DV device. note: This of course is DV but displays as 16:9
There are minor synch issues with displaying to externals this way and they need little workarounds.
I also have a Black Magic Card but rarely use it. Too many issues.