If the modem is hard set to IRQ3, then go into your system BIOS and reserver IRQ3 for the slot it is in (Legacy if ISA-16bit, PCI slot# if PCI modem). Then the software may find it. ;-)
Just remember that COM1shares with COM3 and COM2 shares with COM4 - you may have to play with other devices (ie, if using a serial port COM1, may conflict with COM3; some modems need a COM1 to work, however ... you will have to try it).