Qiyas: It is reasoning by analogy.
Istihsan: Means juristic preference i.e. application of discretion in juristic opinion, preferring one of the two possibilities for greater public good. Istislah means a decision which will serve the public good even though such a decision is not mentioned in the Quran or Sunnah.
Urf: Urf is the common practice among the Muslim community (Ummah) as their common habit or a’ada. The ulema of the Arab countries have applied this method in the evolution of Islamic Law in their respective countries, e.g., in Egypt, Syria, etc.