#if ( $2 = 3 ) plot 'out.txt' using 1:3
#plot 'out.txt' using ($1):($2==3 ? $3 : 1/0)
#plot 'out.txt' using 1:3 t '' with points
#if ( $2 = 3 ) plot 'out.txt' using 1:3
#plot 'out.txt' using ($1):($2==3 ? $3 : 1/0)
#plot 'out.txt' using 1:3 t '' with points