I have had a problem with the bottom circuit

I tried to use Thevenin's theorem to find out, what is the value of current flows past through the 3ohm resistor

I used it with the top circuit and it worked , but when i tried to use it with the bottom circuit , it doesn't work at all.

I dont know what I've made a mistake.

I got Vth = 4.8 V from KVL ---> 8 - 6I - 4I = 0 ( the 3ohm resistor was shorted ) and Rth = 3

So 4.8 - 3I - 3I = 0 so the current should be 0.8A

but when I wrote the circuit into the program, the current that was calculated from the program is 0.888..A

and then I used mesh method to solve it and I got the same answer as the program.

I cant find out what I've made a mistake, Could you tell me pls.

p.s. sorry for my bad english, It's my 2nd language.