Three Letters After the Plural Alif

Diptote Causes — Ultimate Broken-Plural Patterns

أَوْ ثَلَاثَةُ أَحْرُفٍ أَوْسَطُهَا سَاكِنٌ، غَيْرُ قَابِلٍ لِلْهَاءِ، كَـ«مَصَابِيحَ».
Translation: Or three letters follow the plural alif, the middle one being sākin, in a form not accepting the final hāʾ, as in maṣābīḥ.

After the alif in مَصَابِيح come ب، ي، ح, with the middle يْ sākin. This is the author's third pattern.