Unless you misunderstood what whomever you spoke to at your LBS said to you, how can an LBS be so ignorant to suggest that "
Cannondale no longer make a triple for that bike"?
Simply stated, Cannondale does not make derailleurs.
YOU need to find a different LBS because, as my good friend
CAMPYBOB suggests, if your bike has a Shimano SORA rear derailleur, then you can replace it with almost any Shimano rear derailleur which has a suitably long rear derailleur cage ... the exception would apparently be the latest-and-greatest 11-speed Shimano rear derailleurs and the pre-1994 (?) Shimano rear derailleurs.
It is not uncommon to use a Shimano MTB rear derailleur with many Shimano drivetrain configurations rather than a Shimano ROAD rear derailleur which has a "long" (GS) cage because it will allow a potentially wider range of gearing options.
I recommend the older, non-Shadow Shimano XT rear derailleurs which have the standard pull vs. "Rapid Rise"/(reverse pull), first ...
Or, a Shimano SLX if you can live with the lack of a barrel adjuster (which can surely be retrofitted on, somehow!) on the Shadow-type rear derailleur.
If there isn't another LBS in your town AND
/OR if you can DIY, then either buy a replacement rear derailleur from EXCEL, COLORADO CYCLIST, PERFORMANCE-NASHBAR, or eBAY ... just to name a few ...
There are other mail order retailers, of course, from whom you can buy components.