GTA:SA came out in 2005 and uses no shaders at all, so a single HD3450 is slower in that game than the very first ever DX9 card: the 9700 Pro from 2002.
GTA:SA has worked in Crossfire since December 2008 but is likely to only be satisfactory at 2002-ish vintage display resolutions such as 1024x768. Furthermore, the only Windows 10 driver doesn't work with CCC so cannot enable Crossfire. Windows XP and 7 should work fine though.
I have to wonder though why you'd opt to use a HD3450 for games, considering even integrated graphics have been faster for many years now. Intel may only update their drivers for a couple years (leading to glitches and rendering errors for newer games), but that game is so old it presumably would just work. And AMD has had faster IGPs and APUs since their 790GX chipset in 2008, which is the same year the HD3450 came out. To add further insult, the HD3300 in that chipset can even hardware decode H.264 videos in Flash today to take the load off the CPU, while the HD3450 can't--despite having the same core only clocked slower.
BTW, MSRP for the HD3450 was $49 in 2008. At the time, the world's best gaming card was the 8800GTX Ultra which sold for $800 and should give you an idea of the relative performance. 9 years is a long time though: $60 today (equivalent to $52 in 2008) gets you a used 750Ti which is just as fast as the Ultra in such old games but much, much faster in new ones--and uses 115w less power to boot.
Your dual HD3450 should work fine to add 4 monitors for desktop use (or to run the only monitors for Vista and 7 which require the same driver for all GPUs).