If Jacob has 18 hours left to finish his homework, Greg has 18-6 = <<18-6=12>>12 hours left to finish his homework.
The total number of hours Jacob and Greg have left to finish their homework is 12+18 = <<12+18=30>>30 hours.
Patrick has 4 hours less than twice Greg has left to finish his homework, meaning Patrick has 4 hours less than 12*2 = 24 hours left to finish his homework.
Patrick has 24-4 = <<24-4=20>>20 hours left to finish his homework.
Altogether, the three have 30+20 = <<30+20=50>>50 hours left to finish their homework.
#### 50