problem in using pci serial port comm3 in vb

As i require two serial ports for project ,i have installed pci to serial card which gives me comm 3 and 4 port, my comm 1 serial port is working good,I have problem using comm3 serial port in vb,vb gives error of ‘variable not defined’
or ‘port already open’ even if it is connected, all settings of baud rate are correct.