The schoolteacher gets a request from a student for a special favor. She grudgingly says yes. She gets a second request from a different student. She says yes again. The third student asks for something. Now the teacher gives a firm no.
The supervisor gets a request from an employee to have Valentine's day off. The answer is yes. The second employee asks to have the same day off. The supervisor grudgingly says yes. The third employee has to work Valentine's day, even if his wife is having her liver removed.
It is the rule of three. The third person to ask to borrow money will always get turned down. The third problem in your house will always be deferred. The third son has to pay for his own college education.
Hence the problem with Yemen. We sent troops to Afghanistan. We sent troops to Iraq. America is overextended now and Yemen will be ignored. We have no more money and no more troops to deal with any more problems. It's the rule of three.