Synchronisation

I'm running into the same issue.

I noticed that my online timings disappeared and tried a cloud restore, it gave me an error message about incompatible versions and a error code, it was #2 if I remember correctly.

It also said to contact support.

I only noticed that I was logged as guest after this.

One way to get around this issue is opening the game through the Google Play Games app.
This is annoying but at least allows for the login to work.
Let's hope the devs fix this.

I am logged into Google Play Games app but RR3 still asks me to log in if I want to play Time Trials and Online Multiplier.  Google Play Games shows my proper in game name and email but I am still Guest when I play on my Android phone through the app.  It's the same when I launch the game without Google Play Games.

This evening after racing for a while I tried something I'd done previously.  I launched Google Play Games and switched to my other account then started RR3.  No "Welcome" along the top but I had all my M$, R$, and Gold.  As usual it asked me to sign in when I tried Time Trial but it would not log in me when I entered my email.  I exited the game, switched accounts back to my normal one, started RR3 and to my shock the Welcome along the top had my username!  I thought..."Finally, things are back to normal."  Well, I was wrong.  Once the game started I went to Time Trial and it wanted me to sign in and the same old "This account already exists on your device".  I force stopped the game by going to my phone settings (apps and notifications on mine), restarted my phone, then started RR3 using the short cut and not Google Play Games.  The Welcome along the top had my username again but I'm still asked to sign in and it won't let me.  When I started the game using Google Play Games I don't even get the Welcome message.  I tried again using the shortcut icon and it still shows the Welcome message but still need to log in once I try Multiplayer or Time Trial.  This is very annoying.

When it asks you to login just click the option to login (In my case I think it says "Register", I'm not using the english version of the game).

That is the problem.  It will not allow me to sign in when I click the sign in button in the game.  When I input my email address it immediately says "This account already exists on your device" and the enter password screen will not appear.  Even when the game starts normally showing me the "Hi there" with my username, once the game has loaded it still asks me to sign in.  It is a vicious circle which gets me nowhere.  I am missing out on getting a little Gold on Time Trial and Multiplayer.  :)

If you are using a google account click the "Register" button.

I am signed into my Google account.  "It will not allow me to sign in when I click the sign in button in the game.  When I input my email address it immediately says "This account already exists on your device" and the enter password screen will not appear."  There is no "register" as it is "sign in" on my phone.

Hi, I'm also having issues linking my account to Google Play Services.

I've followed the instructions for possible solutions, but nothing works. The game redirects me to the Google Play Services authentication screen. When I enter my details, it says I'm already logged in on the device (obviously), and if I enter another email address, it repeats the process.

I've already cleared the cache and data, restarted and reinstalled the game, and reset the phone, and the problem persists with Google Play Services. And it's the only game on my phone with this issue.

On the other hand, I have no issues with my saved Facebook session; I've been able to save and load the game with my Facebook data. 

Try opening the game through the "Play Games" app not the regular "Google Play" app.

The shortcut icon is called "Play Games".  The options in it are "Play Games" and "Choose Profile".  When I select my profile it says I will be signed into the game as my username, but RR3 still won't sign me in even when the opening screen shows my username.