Com3 does not go missing.;-) It is a virtual port created off com1 and com2.
What I would do is enter device manager again and delete any reference to modem or simple comunications controller. Also look for any yellow ! marks. Delete these to.
Reboot the computer, with the modem driver in. It will find new hardware and point the wizard to the modem drivers. You should be up and working then.
p.s. you don't mention which operating system you are using. This makes a big difference.