The Yongnuo YN568EX II flashes have built in optical receivers that operate on Canons optical wireless system that uses very short pulses of light to communicate. You need an optical Master flash (such as another YN-568EX II) or Canon compatible wireless optical controller (such as the YN-ST-E2) on the camera to control them wirelessly without attaching additional radio receivers to the flashes.

You also need line of sight between the commander and the receiver on the remote flash. This works pretty well in dark studios or even moderately bright indoors areas. It is much more limited outside in bright light that interferes with the optical communication protocols of optical systems that were used by Canon and Nikon for many years. There are other limitations of optical communication compared to radio communication as well.

The advent of reliable and cheap third party radio triggers finally forced Canon and Nikon to transition to radio communication with their latest flash systems. (It also made Pocket Wizard, who make expensive and unreliable radio flash controllers, irrelevant.) Canon introduced their first RT flash, the 600EX-RT, in 2012. Nikon didn't follow suit with their first radio controlled flash, the SB-5000, until 2016.

The best way to control your two YN-568EX II flashes off camera with full HSS and E-TTL capability is to place a YN-622C II receiver on the foot of each flash and use a YN622C-TX transmitter on your Canon camera's hot shoe. If you later decide to add another flash, the YN685 has a built-in radio receiver that is compatible with the YN622C-TX. (But the YN685 has no optical wireless capability at all.)

If using one YN568EX II on the camera to optically control the other off camera flash has worked well so far in the situations in which you shoot, a cheaper option would be to use a Yongnuo YN-ST-E2 if you can still find one for sale somewhere (pretty much everyone has moved on to radio triggering for off-camera flash). This would eliminate the need for two receivers and the associated batteries, but would leave you still controlling the flashes optically. Keep in mind that as you start to use modifiers, such as a softbox that may require you to bury the flash inside it, the line of sight limitations of optical control will probably convince you to eventually move to radio anyway.