First gear and second gear work just fine but when you shift into third gear the atv will slowly decelerate. I can’t figure out what is wrong. Anyone know?

 

The engine and transmission has to work harder in higher gears, have you tried it uphill in second, as this makes it work harder too? The wrong fuel/air mixture will make an engine die when pulling hard and a slipping clutch will be more noticeable when pulling, but then the revs would rise but the bike not go any faster. Usually with the carb, it will just need cleaning out if running weak, or the air filter and choke checking if it is running rich (plug nose colour will tell you if it is rich or weak). Conventional multiplate clutches can be adjusted, but bad slipping can glaze them and once adjustment is used up, new clutch plates are required.

 

So would a new carb help? I’ve already tried to clean the carb but it didn’t help. The machine is on its second carb.

 

New carbs are not always "plug and play" as the air/fuel ratio may not be set right for your machine. Check that spark plug nose colour, to see if it is running rich or weak. If it is neither, it probably isn't a carb problem, Again, you need to know the difference between slowing due to a slipping clutch and power loss. If it is power loss and the carb is OK, check the compression, though low compression usually gives bad starting before any noticeable drop in power.