You can use the .add() method to place a function in the timeline where ever you like.
.to('.col-box:nth-child(3)', 1, {opacity: 1,delay: 0.2}, 0)
.add( function(){ console.log('Woohoo!') } )
.to('.col-box:nth-child(4)', 1, {opacity: 1,delay: 0.3}, 0)
.to('.col-box:nth-child(5)', 1, {opacity: 1,delay: 0.4}, 0)
http://codepen.io/sgorneau/pen/vgxjQb?editors=1111