I really enjoyed the third game, everything was improved from the second. The ending did leave the series open, but I think they did that so they could possibly make a sequel were you go off and fight the Chimera and push they out of North America and possibly out of Europe too (although this is hinted at the end of Resistance Retribution).
And for darkknight, they stopped converting humans because they made a vaccine to the Chimeran strain between R" and R3 :)