This will be the Three maidens grave in the garden of the mosque (I count four, maybe I made a mistake in attribution).

The Üc Kızlar Türbesi (or: Hanımkızlar Türbesi / Yılanlı Türbesi) is estimated to date from the 14th century. This mausoleum is 5 x 5 m (inside dimensions); in the walls, which are 0,9 m thick, there are seven windows.
It is not known who commissioned it, and for whom; neither why there are four cenotaphs/sarcophagi inside.

Correspondent: J.M.Criel, Antwerpen.
Sources: ‘Vakıf Abideler ve eski Eserler (volume III)’ - Vakıflar Genel Müdürlüğü, Ankara 1983
& Website of ‘lifeinbursa.com’.