Mozart's 'Requiem' was performed at the funerals of Joseph Haydn (1809), Ludwig von Beethoven (1827), and Frederick Chopin (1849). What else do all three funerals have in common?

Their own compositions (Funeral March/Requiem) was also played at their funerals.

Mozart's "Requiem" was played at Chopin's funeral and along with Chopin's own "Funeral March", one of the most famous funeral compositions ever written.
[queenshall.org/ellen_annor-adjei.htm]
http://www.worldofopera.org/component/k2/item/852-episode-153-chopins-funeral-mozart-requiem-aeternam

During the funeral, Michael Haydn's "Requiem" was played and at a memorial service on June 15, 1809, they played the "Requiem" written by Haydn's dear friend Mozart.
http://www.midiworld.com/haydn1.htm

For his part, Beethoven offered other contemporaneous examples in his works, including the "Marcia Funebre sulla morte d’un Eroe" (“Funeral March on the death of a Hero”) from his A-flat-major Piano Sonata (Opus 26, of 1801), which would be played during his own funeral procession a quarter of a century later.
https://vanrecital.com/tag/ravel/

I see you're up to your usual high standards of research, Gt. And your answer is certainly correct. But just to keep up to my expected level of contrariness, it isn't the answer I had in mind.

The same singer who sang at the funerals of Beethoven and Haydn sang at Chopin’s funeral but as yet I cannot put find a name

The singer who performed at all three funerals was Luigi Labanche (or Lablanche)(1795-1858). Labanche began as a famous boy soprano and went on to become one of the greatest bassos of the 19th century. He had a two octave range and had once been singing teacher to the young Queen Victoria.

At the funeral of Beethoven in 1827 he was one of the thirty-two torchbearers who surrounded the coffin, and he also sang in Mozart's Requiem. He sang the same Requiem at Chopin's funeral in 1849. He was the soloist, singing a Lacrimosa, at Bellini's funeral in 1835.

https://en.wikipedia.org/wiki/Luigi_Lablache