Captaining, Haines led from the front with 59, while Clark fulfilled his promise with his maiden first class 100 for the county, before paceman Atkins followed up with the two prized wickets - of former England player Ben Duckett, and Notts skipper Mullaney, whose 192 was by far the highest individual score of the match.
This is a big season for all three of these three Sussex players, with their impressive performances coming as no surprise to those witnessing their development on their journeys through club cricket and into the county set up.
From fearlessly taking on wily, experienced Sussex Premier division bowlers, now tall, left hand bat Tom Clark has blossomed both physically and in patience and technique, his mature innings at Hove delighting the Sussex faithful.
His county colleague and Horsham coach Will Beer said: “You absolutely love to see it. Another young Horsham player coming through the ranks! A serious player and so well deserved! This is only the beginning for him.”
Sussex visit Derbyshire from today.