I'm an American citizen, and I was hoping I could get some help on visa requirements.

I really only have a layover in Beijing, and I'll be spending a couple of days in HK. This is my projected flight schedule:

Aug. 4: Flying USA->Beijing (3 hour layover); then flying Beijing->HK.
Aug. 4-7: staying in HK
Aug. 7-19: traveling in Thailand.
Aug. 19: Flying Thailand->HK
Aug. 20: Flying HK->Beijing (15 hour layover); then flying Beijing->USA

I'm just wondering what my requirements will be in mainland and Hong Kong. I understand that I won't need one in HK because I'm staying for less than 90 days, but will I need to get a Chinese one for my 3 hour layover going/15 hour layover leaving? Will the requirements change depending on whether I decide to leave the Beijing airport during my layover? Any help would be appreciated. Thanks.