Is the 30 teeth chainring on Campagnolo Mirage 52/42/30 crankset replaceable?
The bolt circle diameter for recent road triples is 74 mm, which allows something around 23 teeth. Looking at the catalog of Specialites TA, their smallest chainring is 24 teeth.

In addition to triple crank, you will need a triple compatible front derailleur and very probably a long cage rear derailleur. There are multiple discussions about chain capacity on this site, search for them.