OK so there are three options for an eyefinity set-up: 1 use an active adaptor to provide the third clock for the third monitor; 2 use a displayport monitor with a built in clock / DAC; and, 3 use a FLEX card which has an inbuilt third clock generator.
The first is the cheapest and most used. I dont quite understand your question, regardless of whether you use DVI-HDMI-DP or DVI-DVI-DP or DVI-DP-DP or whatever combination the above still holds. So in any of the combinations I listed above you will still need an active adaptor to connect the third screen.
could i connect the dvi's to the HDMI with the adapters as this would make buying monitors cheaper
That wouldn't affect the cost of the display only the size would. I'd second the recommendation of using a Sapphire Flex card - you can add multiple displays in HDMI or DVI (with DP adapter) with relative ease.