On Monday, Shawna was short of 30 - 12 = <<30-12=18>>18 situps
On Tuesday, Shawna was short of 30 - 19 = <<30-19=11>>11 situps
On Wednesday, Shawna would have to do 30 + 18 + 11 = <<30+18+11=59>>59 situps
#### 59