Well you certainly can connect one of them to the TB port of your iMac.
If one of the external monitors was a TB monitor then you could chain them (iMac --> TB display1 -->> display2). But if it's the older mini-displayport version then no, there is no straight forward way to connect a third monitor, not without additional hardware.
There is a device called the Matrox DualHead2Go which allows two monitors to be combined as an extended display. I'll leave it as an exercise for you to figure out the appropriate mini-displayport adapters to use that thing.