I do knows about that Hazrat Abubakr(RA) and Hazrat Umar(RA) are burried next to the Prophet Muhammad (PBUH) but I heard there is one one grave is there which is for Hazrat Isa (RA) when he will return to the world again.
“Then he will die and be buried with me, in my grave (i.e. very close to Rasulullah’s grave).
On the Day of Qiyaamah, Isa and I will rise from one grave between Abu Bakr and Umar.” Narrated by Ibn Jauzi in Kitaabul Wafaa.
[Mishkaat, page 480]