Chromatin is the lose form of genetic material & the "normal state" of DNA, RNA, and protein in the nucleus and in the cells.
Chromosomes are a condensed form of chromatin. Chromatin becomes chromosomes in the Prophase of Mitosis and Prophase 1 and Prophase 2 of Meiosis. It loosens again in the Telophase in Mitosis and Telophase 1 and Telophase 2 in Meiosis.
Sister Chromatids are made when chromosomes replicate during the cell cycle. The replication leads to the formation of the chromatids. The chromatids are joined by a single centromere. They separate during Anaphase 2 of meiosis.
I hope that this helps! Sorry if it doesn't I tried my best to explain!