I know this thread is old, but this is just for the record.
I have the above motherboard (I will be replacing it very soon but regardless....) and Crossfire does work on it even though it is not officially supported. The first card will work at x16 in PCI-E 2.0 specification and the second in x4 PCI-E 2.0 (equivalent to PCI-E 1.1 @ x8 speed). Depending on the application there is no bottlenecking.
In ShaderToyMark (a extremely GPU taxing DX11 benchmarking tool) my system achieves (@ 1920x1080, no AA) only 45 (7 frames per second) points without Crossfire Enabled. With Crossfire enabled I get 94 points (15 frames per second) or a 208% increase!!! notice the low fps and you see what I mean about it being taxing.
Now I would imagine with Anti-aliasing on, the second card running on PCIE VERSION 2.0 x4 would become a constraining factor, but ShaderToyMark does not seem to have the option to enable AA at this stage.
Personally I would like to make the extra bandwidth available so i've got an ASUS M4A89GTD USB3 mobo on the way, So i can run both cards at PCI-E 2.0 x 8.
To summarise, it is possible to run Crossfire on a Gigabyte 870A UD3 motherboard, however depending on the game or application and the settings (particularly Anti-Aliasing), your results may be constrained by the PCI-E 2.0 x4 slot.