I'm Northeast...was seeing DT in Providence and Boston during the 90's. I've not ever heard of the Ytse Cons.
I think the 3rd one was the last one. They were held in 1995-1996, arranged by a guy named Jeff Chew. Primarily, the people who attended were part of the Ytsejam Digest, a DT IRC forum from the early internet days. Here's a post from the Digest I found from about a month before the YCIII:
Dear jammers,
Hey there! Here's an update for the YtseCon III! First of all, it will
take place at the La Guardia Marriot (Queens, NY), on Saturday, October
12th from 1pm to 5pm. We have spoken to Dream Theater and management, and
they say that there is a good chance that they will be there.
The cost for pre-paid tickets is $20 per person. The cost at the door
will be $25 per person. This will cover the cost of the conference room
and other miscellaneous stuff (name tags, programs, etc.). If you would
like to pay in advance, please send a check to:
YtseCon III
c/o Jeff Chew