physical interface between Cyclone III & DAC

The DAC can be directly connected to FPGA 3.0V or 3.3V single ended IO pins, as e.g. the said THDB_ADA board does. The C3 starter kit exposes IOs only through the HSMC connector, so you need to make an adapter that connects your eval module to this interface.

P.S.: All C3 starter kit IOs are in fact 2.5V supplied, so you have to check, if the eval module can be driven by this level, otherwise you need level converters.

Interestingly, the THDB_ADA board also interfaces the 2.5V FPGA IOs directly, although the respective requires even a 2.1V Vih level (DAC5672 2.0 V). A good circuit layout would be required to achieve a reliable signalling under this circumstances.