Q: Transmission is skipping 3rd gear

Hello - a "skipping a gear condition" like this is most commonly a failure of the 3-4 gear clutch pack (for the 4L60E transmission). It is also possible that one of the valve passages in the transmission is damaged, preventing proper solenoid/valve operation to engage the clutch pack. I would recommend having a transmission diagnosis to confirm the failure point. A mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment of damage and cost estimate for repairs

Hi there. Is the check engine light on? Are there any stored codes that may lead us to a specific system? Does the transmission have the correct amount of fluid? Aside from the electrical and any codes stored, there could be a couple of other causes: a sticking detent cable or a sticking valve in the transmission valve body. The detent cable is what connects the throttle body to the transmission and aids in transmission shifting based on throttle position; vehicle speed and other electronic factors contribute as well. If the cable is sticking, it could be affecting the transmission shifting. The cable can be lubricated to a point as removing the cable from the transmission usually involves removing the pan and valve body to gain access. Open the throttle, then clean and lubricate the exposed cable. Move the throttle to fully open several times to work the lubricant into the cable sleeve. Test drive the truck and see if there is a change. If there is no change, the cable may not be repairable or there is a more involved underlying issue. I strongly suggest having a qualified technician perform an inspection to avoid replacing unnecessary parts. Your Mechanic has several available technicians that can assist you with a transmission does not shift properly inspection.