If you are looking for a dedicated inexpensive remote control for your Sony a7III to take some pictures without the fuss of starting apps etc. then you might consider this remote control which has been developed by a third party company (BV & Jo).
The remote control is quite handy if you want to use the bulb function of the camera in manual mode. The shutter opens once you push the S-button and closes once you push the S-button again. The remote control is also super small (below compared with an iPhone 4S).
Before you can actually use the remote control with the a7R you have to change the remote control setting on your camera.
You can get the a7III remote Control on Amazon