If he gathers for 2 hours, the number of minutes gathering is 2 * 60 minutes per hour = <<2*60=120>>120 minutes of gathering flowers
Gathering for 120 minutes / 10 minutes per flower = <<120/10=12>>12 flowers gathered
If he gathers 12 flowers initially but lost 3, he has 12-3 = <<12-3=9>>9 flowers gathered and kept
Since he has 30 classmates total and has 9 flowers, he needs to gather 30-9=<<30-9=21>>21 more flowers
Assuming it takes 10 minutes per flower, it will take him 10 * 21 = <<10*21=210>>210 minutes to gather the remaining 21 flowers needed
#### 210