I have the Cooler Master MasterBox TD500 case a gtx 1060 3gb at 100% fan speed and in far cry 5 it's reaching almost 80 degrees celcius and the temperature is whats making me lag so bad and I did put new thermal paste on it
Yes, that case has plenty of air flow for that card. That's not the problem. And replacing the TIM shouldn't have been necessary already. Did you buy the card new or used?
Does the fan reach 100% with stock settings, or did you adjust the fan curve to run at 100%?
I bought it used and the fan I keep at 90% but 100% sounded better
75-80C is not a problem with modern cards like the GTX 1060 if those temps are when the GPU is at or near 100% usage. But, unless the loud fan noise doesn't bother you, why not set it back to its default and see what the temps are?