ls -t | sed -e '1,3d' | xargs -d '\n' rm
What’s going on here?
ls -t
lists all files in the current directory in decreasing order of modification time. The most recently modified files are first, one file name per line.
sed -e '1,3d' deletes the first 3 lines, the 3 newest files.
rm collects each input line (without the terminating newline) and passes each line as an argument to rm