There are three possible ways to go here...
1. There are some cheap add-on boxes that allow you to plug in a third monitor. They sort of behave like a "Y" cable, in that they only duplicate what is already shown on one of the other monitors. You may not be able to actually display independent content unless it specifically says so. So, be careful what you settle for.
2. You may be able to add a second cheap (perhaps $40 US) video card beside your existing one. That is, if the machine has an available slot to plug it into the motherboard. However, you should have a Mac expert advise you on this because I am a PC guy.
3. If no slot is available then you probably need a video card that is capable of running three. (If you don't already have one.) Many of the more modern high-end ones can, but you need to be sure. It is usually mentioned in the product description. Some of them come with a specially designed "Y" cable and software to make it work correctly.
