1) The first one has a line over we call it fatha, it makes ث sound as Tha.
2) The second one has a loop/circle over it we call it dhama, it makes ث sound as thu.
3) The third one has a line under it we call it kasrah, it makes ث sound as thi.
REMEMBER: IN THIS LETTER WE PRONOUNCE THE THA LIKE IN THE WORD THINK.