Brihat Parashara Hora Shastra
मध्यांशो मध्यखेटः स्यादुपखेटः स एव हि । विलोमगमनाद्राहोरंशाः शोध्याः खवह्नितः ॥ ५॥
In that case these three are called Anthya Karak, Madhya Karak and Upakheta. In the case of Rahu deduct his longitude in that particular Rāśi from 30.
english translation
madhyAMzo madhyakheTaH syAdupakheTaH sa eva hi । vilomagamanAdrAhoraMzAH zodhyAH khavahnitaH ॥ 5॥
hk transliteration by Sanscript