I know that ATI.com says that the Radeon HD 5450 supports three displays. My question is, how does it support three displays without DisplayPort? I see only VGA, HDMI, and DVI on 5450s.
This is from the AnandTech review of the AMD Radeon HD 5450:
Finally, since the 5450 is a low-profile card, we have yet another variant in the port configuration. Our sample card is equipped with 1 DVI port and 1 DisplayPort directly on the card, and a VGA port on the bracket attached to the card via cable. The 5450 – like all other 5000-series cards – supports 3-monitor Eyefinity, so all 3 ports can be used at once, although the VGA port is obviously going to make it less desirable here.
We should note that only cards with a DisplayPort will support Eyefinity, which means that if a vendor decides to equip the card with an HDMI port instead, they have to give up Eyefinity to do so. We expect to see some cards with HDMI for the HTPC crowd, but at this point we don’t have any idea as to how many there will be. DVI-to-HDMI adaptors still work here, so we may see vendors spend a bit more money and go that route instead.
I've highlighted the part you'll be interested in. Basically, without DisplayPort, there's no Eyefinity and, therefore, no 3 monitor support.
You have to get a Radeon 5450 with a DisplayPort in order to do three monitors. They do make them, but you have to be careful not to buy the wrong one.
Also, if you do not have to a DisplayPort monitor, you need to buy an approved adapter or the third port will most likely not work (it sometimes still does with other adapters).