Cctld / subdomain / folder, which is preferred for an e-commerce website?

Hi

I run a website which sells products to multiple countries primarily through pay per click advertising.
I want to expand the reach of the website as well as improve the SERP.

If I have a CCTLD for each country, that’s great for the PPC advertising, but is pretty rubbish for SEO, as each site is individual.
if i have a generic .com/UK/ .com/US/ etc that I assume is better for SEO, lots more pages etc, but not as good for PPC as people prefer to click on country specific domains(…? is that true?)
The third option is to have subdomains UK.domainname.com, which doesn’t really tick either the PPC or the SEO box.

Does anyone have any advice as to which route below is best when considering PPC and SEO together?

  1. lots of CCTLDs
  2. domainname.com/country
  3. country.domainname.com

Thanks!