[Samsung Gear S3] Jackz314:
You're not alone, I've contacted Samsung support several times about the incompatibility issue in their app, but it seems like nobody in Samsung cares, I either don't get responses at all, or they just tell me to wait for updates (which would probably take years considering how Samsung handles this kind of stuff.

BTW, in case you're wondering what went wrong, here's the error log I retrieved from Logcat:

** --------- beginning of crash
03-28 19:53:29.528 3377-3377/? E/AndroidRuntime: FATAL EXCEPTION: main
*** Process: com.samsung.android.app.watchmanager, PID: 3377
*** java.lang.NoClassDefFoundError: Failed resolution of: Landroid/content/pm/IPackageInstallObserver$Stub;
******* at com.samsung.android.app.twatchmanager.packagecontroller.PackageControllerFactory.makeInstance(Unknown Source:6)
******* at com.samsung.android.app.twatchmanager.packagecontroller.PackageControllerFactory.getInstaller(Unknown Source:4)
******* at com.samsung.android.app.twatchmanager.util.CleanupAsyncTask.onPreExecute(Unknown Source:40)
******* at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:648)
******* at android.os.AsyncTask.execute(AsyncTask.java:595)
******* at com.samsung.android.app.watchmanager.setupwizard.HMConnectFragment.startCleanupTask(Unknown Source:28)
******* at com.samsung.android.app.watchmanager.setupwizard.HMConnectFragment.access$3100(Unknown Source:0)
******* at com.samsung.android.app.watchmanager.setupwizard.HMConnectFragment$13.handleMessage(Unknown Source:121)
******* at android.os.Handler.dispatchMessage(Handler.java:106)
******* at android.os.Looper.loop(Looper.java:164)
******* at android.app.ActivityThread.main(ActivityThread.java:6656)
******* at java.lang.reflect.Method.invoke(Native Method)
******* at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
******* at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)
**** Caused by: java.lang.ClassNotFoundException: Didn't find class "android.content.pm.IPackageInstallObserver$Stub" on path: DexPathList[[zip file "/data/app/com.samsung.android.app.watchmanager-ExhBUk2ofzT0Hwta2wDw9w==/base.apk"],nativeLibraryDirectories=[/data/app/com.samsung.android.app.watchmanager-ExhBUk2ofzT0Hwta2wDw9w==/lib/arm64, /system/fake-libs64, /data/app/com.samsung.android.app.watchmanager-ExhBUk2ofzT0Hwta2wDw9w==/base.apk!/lib/arm64-v8a, /system/lib64]]
******* at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:125)
******* at java.lang.ClassLoader.loadClass(ClassLoader.java:379)
******* at java.lang.ClassLoader.loadClass(ClassLoader.java:312)
******* at com.samsung.android.app.twatchmanager.packagecontroller.PackageControllerFactory.makeInstance(Unknown Source:6)*
******* at com.samsung.android.app.twatchmanager.packagecontroller.PackageControllerFactory.getInstaller(Unknown Source:4)*
******* at com.samsung.android.app.twatchmanager.util.CleanupAsyncTask.onPreExecute(Unknown Source:40)*
******* at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:648)*
******* at android.os.AsyncTask.execute(AsyncTask.java:595)*
******* at com.samsung.android.app.watchmanager.setupwizard.HMConnectFragment.startCleanupTask(Unknown Source:28)*
******* at com.samsung.android.app.watchmanager.setupwizard.HMConnectFragment.access$3100(Unknown Source:0)*
******* at com.samsung.android.app.watchmanager.setupwizard.HMConnectFragment$13.handleMessage(Unknown Source:121)*
******* at android.os.Handler.dispatchMessage(Handler.java:106)*
******* at android.os.Looper.loop(Looper.java:164)*
******* at android.app.ActivityThread.main(ActivityThread.java:6656)*
******* at java.lang.reflect.Method.invoke(Native Method)*
******* at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)*
******* at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)*
****
****
*** --------- beginning of system (end of crash)

Seems like a stupid coding problem to me, and a fix would be pretty easy by just changing several lines, but unfortunately they don't seem to worry about problems in their apps, so... The choices are to wait, switch to Android Wear, or roll back to Android O.