Garage ceiling fan / light - changing switch
The old smart switch is not capable of controlling a fan and light separately... and you never claimed it did.  That means your 3 cables are 


Supply (always-hot)
Onward (always-hot) power to something else
(Switched-hot) power to the fan/light


First, all grounds get nutted together with a pigtail that you'll need to buy, and get pushed into the back of the box (except the pigtail, bring that out!)   

Second, all white wires get nutted together and pushed into the back of the box.  These are neutral.  You don't need a pigtail here; dumb switches don't use neutral. 

Third, that switch goes into the trash.  Get a plain single switch.  

Fourth, The ground pigtail goes to this switch's ground terminal. 

Fifth, get a black pigtail and attach it to one of the screws on the switch.  This gets wire-nutted together with the black wire you are fairly sure is supply hot.  Feel free to wire-nut this very lightly, you'll be adding to it later.   These two wires are called always-hot.

Sixth, pick an unattached black wire.  Attach it to the remaining screw on the switch.  We are guessing here.  

Seventh, power up the circuit and see if the fan/light is starting to work.  If both work, awesome, you have identified switched-hot for both light and fan... go to step 8.   If neither one works, repeat step 6-7 with the other unattached wire.   If only one of them works, power down and repeat step 6-7 with the other unattached black wire.


  If one wire works the fan and the other works the light, then you've made a monkey out of me.  Fish that double switch out of the trash and follow somebody else's instructions.  


If both of them work with one wire attached, then my advice stands.  Leave that wire attached and step 7 is done.   

Eighth, take the remaining black wire and attach it to the black wires under the wire-nut.  This time, crank it down gorilla-tight.   

For bonus points, identify the light+fan wire and mark it with red tape.   Or if they are separately controlled, mark the fan with red tape and the light with blue tape.  You'll thank me later.  If you had color coded these wires at the very start of this, life would've been much easier.  Colored tape is your friend.