The Apple articles tell you the path Apple has laid out for your success, but not the entire picture.
To drive three displays off these cards, you need all converters to be ACTIVE, POWERED converters. The signal levels must be re-driven, and the Driver circuits to do this cannot quite get enough power off the card itself to do this.
This article on AMD/ATI web site has an additional descriptions and "Rules" in the paragraphs at the top of the page. The tables below list some qualified adapters under the correct headings.
Read Carefully: DisplayPort is not MINI DisplayPort, and there are headings for ACTIVE as well as PASSIVE included.
Also note that Amazon searches do not always return EXACT results.
"Dual-Link" contain double data signals, for servicing SINGLE displays that are wider than 1920 wide.
The Mac is completely capable of Mirroring displays in any combination on the same Display card.
The USB-to-DVI box BDAqua cited is an interesting way to get an additional display output, but is probably not needed since your 5770 can already support three displays if you use the right adapters. It is said to be s-l-o-w.
