ثلَثَ يَثلُث ويَثلِث، ثَلْثًا، فهو ثالِث، والمفعول مَثْلوث

أخذ ثُلُثه، أي جزءًا من ثلاثة أجزاء متساوية.

🇬🇧 Translation:

He took its third, i.e., one part out of three equal parts.

أخذ ثُلث ماله.

🇬🇧 Translation:

He took a third of his wealth.

صار ثالثهم، كمَّلهم بنفسه ثلاثة.

🇬🇧 Translation:

He became their third, making them three by joining them.