What do you plan on using the 3 monitors for? Eyefinity gaming setup? Every day 2D use?
With one 5000 series card you can use 3 monitors, any combination for the first two can be either DVI DVI or DVI HDMI the 3rd montior has to use the DisplayPort output. Which unless you have a DP monitor will cost you either $100 for an active DP to HDMI/DVI adapter or $25 for a DP to VGA active adapter.
All of your options 1, 2, and 3 are possible. Though with #2 id go with a 5670 or lower for the second card, assuming it wont be supporting any monitor with a heavy 3d task set to it. And option 3 might not be completely necessary unless you are running a main 2560x1600 monitor or at least a 1080 Eyefinity gaming setup. #3 will drive 3 monitors with crossfire enabled but all 3 have to be connected to the first card, I believe.
well i've been using 3 22" monitors with a 5770 (SINGLE) and it has worked fine. although i've had to turn down some settings to med from high... not really a big deal for 5040x1050 IMHO. games look perfect! BUT i recently bought a 5870 to power my graphics addiction... i've been getting alot of GSOD's tho... tried everything on the internet to fix it... but still get em. When my 5870 is actually running and not GSOD'in me games are awesome! i play crysis, warhead, lfd2, BFBC2, GTA4, all other games at right around 30-40fps at 5040x1050 all on max settings (not AA of AF tho) but at that res u dont need it.
So i'm back to my 5770 for now... it is slighly OC'ed 870/1250. games look good tho... just not as nice as the 5870 lol. If you are going to buy all of this up front and money is no problem, get your monitors and a 5870/5970, but if money is tight get your monitor and a 5770... you can later CF it with another when 10.3 drivers come out... hope this helps!