I see no reason you cannot use 2x DVI -> VGA adapters for two of the monitors and one DP -> VGA adapter for the third monitor.
Although several people have posted that VGA "looks crap" on LCD, I have to say that, in MY case, I really cannot tell any significant difference in image quality between the two DVI screens and the one using the VGA input.
Even further, you SHOULD be able to even use one HDMI connection, one DVI, and one VGA in Eyefinity. The main thing is getting the right adapters that work properly.
I'm quite sold on the Monster DP -> VGA adapter I got although I paid $50 USD for it off the shelf at Fry's. It has worked for several weeks for me without any significant problems at all.
+1 This.
I personally am using 3 VGA adapters atm. No hiccups at all.
DP->VGA adapter range from very expensive to very cheap. I haven't seen people have any real issues with anyone of the adapters, no matter the price.