transmission problems (automatic) 2.5 turbo on 1990 Mercedes-Benz 300D

has no problem shifting thru 1st & 2nd but doesn't always hit 3rd. fluid level ok.

Asked by for the 1990 Mercedes-Benz 300D
how many miles? was trans serviced regularly?
could be a kickdown cable, kickdown switch, a vacuum issue, plugged up filter or just a worn out transmission. not that difficult to check if you want us to.