If it skips the middle ring when downshifting from the big ring, the problem is likely in the shifter. There is a ratchet with spring loaded pawls in the shifter and those pawls can get gummed up. Liberally hosing out the shifter body with WD40 or other thin solvent/lube can sometimes free up the pawls and restore proper operation. Mtb shifters are usually fairly well sealed but access to spray in the WD40 can be had by taking off the cover on the cable replacement port.