There seems to be a lot of confusion about tri-sync monitors although I have tried to explain on our ArcadeVGA pages on our site. To summarize:
If you connect a 15-25-31 Khz monitor to a normal VGA card, and set to 640 x 480 60Hz it will work. BUT if you do this, why did you pay the extra for a multi-frequency monitor? You are using it as a VGA monitor unless you have some method of also generating 15Khz and 25Khz modes. The reason for a multi-frequency is that it can be used as an arcade monitor, standard resolution, as well, at 15Khz which is not VGA, and medium-res which is 25Khz.
The other factor is, if you use it with the ArcadeVGA card it will default to 15Khz all the time as the card will not know it is a multi-frequency monitor. It will assume by default it is a 15 Khz standard-res arcade monitor so it will run all resolutions at 15Khz. Using a multi-frequency monitor the resolutions above 640 x 288 will run non-interlaced as opposed to interlaced (with flicker) on a standard-res monitor. The ArcadeVGA card can be configured to optimise a multi-frequency monitor.
Andy