What (travel) affiliate program(s) are these sites using?

Hi,
You know those travel sites where you can compare hotel booking prices? e.g. www.simonseeks.com
These sites get their hotels/prices from different booking sites, such as: expedia.com, booking.com, hotels.com etc.
Can anyone tell me if there is one affiliate program where you can promote all of these sites, or do I have to sign up for each site individually? Because when I searched here on DP someone mentioned it’s not allowed to promote multiple booking sites on one site, or am I wrong?.. Though many sites out there are doing this like I explained above so it must be possible.
Anyone with experience that can answer my question? I’d appreciate it!
Thanks

See original post by Frankve