J = japanese, X = other car type 1, Y = other car type 2
There needs to be 2 types of normal cars and 1 type of Japanese car.
XXX
XXY
XYX
YXX
XYY
YXY
YYX
XXJ*
XJX*
JXX*
XJJ*
JXJ*
JJX*
XYJ*
XJY*
JYX*
JXY*
YJX*
YXJ*
YYY
YYJ*
YJY*
JYY*
JJY*
JYJ*
YJJ*
JJJ*
Those are the 27 combinations you could see. Remember that X and Y are both any other car but I put two of them there to show how there's a 2/3 chance of a non-Japanese car and a 1/3 chance of a Japanese car.
Out of those, there are 19 situations where you'd see at least one Japanese car.
Mathematically:
You have 3 cars on the road that you see. You need to consider the situations where you have either 1 J car, 2 J cars, or 3 J cars.
For the 1 J car, you need to choose which car is the J car. You do this with (3 choose 1) = 3:
??J
?J?
J??
For the 2 J cars, you choose which 2 are the J cars. Use (3 choose 2) = 3:
JJ?
J?J
?JJ
For the 3 J cars, use (3 choose 3) = 1:
JJJ
Now you use the probability part. For each of the 3 possibilities with only 1 J car, the probability of the car being J is 1/3 and the probability of the car not being J is 2/3. So with 1 J car, you use (1/3) * (2/3) * (2/3) = 4/27. Multiply this by 3 because there are 3 ways the J car could appear (as shown above). You get 12/27.
For the 2 J case you use (1/3)(1/3)(2/3) = 2/27. Multiply by 3 because there are 3 ways to find 2 J cars in a group of 3 cars. You get 6/27.
For the 3 J case you use (1/3)(1/3)(1/3) = 1/27. Multiply by 1. You get 1/27.
Add them up. You get 12/27 + 6/27 + 1/27 = 19/27.
For the 1 J car case:
JXY
JYX
XJY
YJX
XYJ
YXJ
XXJ
XJX
JXX
YYJ
YJY
JYY
Those are your 12/27.
Notice how if we hadn't paid attention to where the J went, we'd have these:
JXY
JYX
JXX
JYY
Those are your 4 cases (out of 9 - didn't put the other 5 without the J).
For the 2 J case:
JJX
JJY
JXJ
JYJ
XJJ
YJJ
Those are your 6/27.
I think the 3 J case is obvious:
JJJ
Hope this helps you visualize it.