If you have 2 drives, you can have the following configurations:
a) One drive is connected as master for each of the two channels,
b) one drive is connected as master and the second as slave to the same channel.
There is no way you can connect a drive both as a master and a slave at the same time.
There is no way you can connect a drive as a slave to a channel which does not have a master.
According to the above, check how many cables are there for HDD connection.
a) If both drives are connected on the same cable, then one is master and the other is slave, and you must connect the third drive as a master to the other channel, using a separate cable (found cheap in computer shops - IDE cable)
b) if each drive is connected through its own cable, then each is connected as a master and you must connect your third drive as a slave to any one channel. Usually there are two connectors on IDE cables, so you should have no problems to connect.
I think that you have at least one CD or DVD which is also connected to the IDE channels. In this case, logically treat it as a HDD for the sake of the argument above.
If you have four devices already connected to the IDE subsystem (2 HDD + 2 CD, etc), then you're in trouble. I read about add-on IDE cards which you install just like other peripheral cards and add 2 more channels of 2 devices each, but I'm not knowledgeable on this and you may have to look around for additional information.
Please note that each additional device introduces a load to your power supply. If I had four IDE devices already, I would prefer to replace one of the HDDs with a larger capacity one, if needed.
Good luck,
Engin