First find the number of complaints per day when the store is short-staffed: 120 complaints/day * 4/3 = <<120*4/3=160>>160 complaints/day
Then find the number of complaints per day when the self-checkout is broken and they're short-staffed: 160 complaints/day * 1.2 = <<160*1.2=192>>192 complaints/day
Then multiply the number of complaints per day by the number of days to find the total number of complaints: 192 complaints/day * 3 days = <<192*3=576>>576 complaints
#### 576