1. Who have you served today?
(I think this tense is used because the content has a relevance as he speaks.). Yes, and because today is also until the moment of speaking.
Why is it not "three thousands"? Study your numerals:-)
About three thousand were added to their number that day. Correct.
Why would one capitalize the underlined words?
Use of this Community Center and its amenities are at your own risk. Matter of choice, really.