There is no adjustable linkage for the range shift lever on my B2650. The shift arm at the tranny attaches with a spring pin to the shift lever.
Question: Does the tranny ever pop out of 3rd range? That would mean the fender cover is interfering with the lever enough that it won't shift positively into 3rd. Repair/adjust time. The only way I could see adjusting it is to remove from the tractor and bend it. Or maybe carefully grind a little fender plastic away enough to clear the lever.
But if you feel all shift ranges engage solidly when shifting, especially 3rd, then just get used to the quirk - or blackout the LMNH labels and re-label.
My shifter has always sat about 1/2" forward of all the labels on mine. I'm now so used to the shifter positions I don't often look anymore to check gear or shift.