A synchronizer problem can definitely cause a grind when shifting. Its purpose is to match the speed of the gears just before the shift collar connects the input and output shafts together. In the case of 3rd. gear on a 3 speed, that collar simply connects the transmission input shaft which is splined to the clutch to the output shaft which is connected to the driveshaft. There's a lot of inertia contained in the spinning clutch disc and input shaft, and several orders of magnitude more involved in the spinning driveshaft, rear axle, and ultimately the whole vehicle. It's the synchronizer's job to spin the speed of the clutch and input shaft up or down to match the speed of the driveshaft to prevent gear grinding, in the few seconds it takes to complete the shift. A worn synchronizer blocker ring, and/or too-slippery lubricant makes the synchronization effort ineffective. The only option otherwise is to match the engine speed to the output shaft by either idling the engine down somewhat, or double-clutching. Double clutching is definitely an acquired skill, one that takes a great deal of practice.
Back in the dark ages when I was driving a rig with two transmissions and a gross weight approaching 40 tons at times, we would work our way up and down through 15 gears without ever disengaging the clutch except for starting, stopping, and backing up! All the shifting was done simply by matching up engine speed with road speed, and selecting the right shifter position (two of them) to slide in and out of gear with no load on the transmission(s). Fun stuff!
Jerry
