If your crankset is very close to the bike frame like in this picture, you either need a longer bottom bracket or to replace your middle chainring with a smaller one.

The bike's body bends during use and it will scratch itself on the ring teeth and the chain. Sometimes older bikes have a bigger number of teeth on the middle chainring like this (s:m:l ring teeth ratio is about 32/48/52 teeth).

numbered bike anatomy graph

chainring = 31; bottom bracket spindle = 26; bottom bracket shell = the hole where 27 is marked to go in

~source~