Collation of donation and procedure to determine legitime

Dear PAO,

My parents donated a 300-square-meter lot to me. I have already developed and introduced improvements to the lot. Three years after my parents' demise, my two siblings relayed to me their intention to partition the 1-hectare farm land which forms part of the estate of our parents. I relayed to them that I have no objection, and I am willing to accept which part of the land will be allotted to me as my share. I was surprised when they informed me that the land will not be divided equally; instead, I would receive a smaller share because they claimed that the land which had been donated to me was an advance of my inheritance. Is there a need for the land donated to me to be added again to the estate of my parents? What are the procedures to be observed in order to determine our accurate sharing in the estate of my parents?