If you've already had the setup working with 3 displays than you neednt worry too much about crossfire. Down below is a quote of what configurations work on multi monitor displays on a 7970.
If you wish to use more than two displays on a 7970, at least one of them must be connected to a DisplayPort port. The following configurations will work:
DVI-D/VGA + DVI-D/VGA + DP
DVI-D/VGA + HDMI + DP
HDMI + HDMI + DP
DVI-D/VGA + DP + DP
HDMI + DP + DP
DP + DP + DP (daisy chain)
As far as crossfire is concerned, its basically putting 2 cards together to use them both to power your graphics rendering.