Cassius doesn’t actually state this as a fraction.
Act I, Scene iii is the part of the play that describes the strange, supernatural storm that is raging over the capitol. It opens with Cicero and Casca discussing the strange sights they’ve seen, but there is no mention of Caesar or a conspiracy.
Part of the way through the scene, Cassius enters and begins talking to Casca. After a brief discussion about the storm, Cassius steers the discussion to Caesar, saying that “Now could I, Casca, name thee a man most like this dreadful night, that thunders, lightens, opens graves, and roars as doth the lion in the Capitol.” Cassius is saying that the storm is a result of and reflection of Caesar’s growing power.
This leads to further discussion of Caesar and the conspiracy against him. Cassius reveals to Casca and Cinna his bit of deception, saying he will send Brutus three letters asking him to join the conspiracy. He tells Casca, regarding Brutus’ possible recruitment,
“three parts of him is ours already, and the man entire upon the next encounter yields him ours.”
We can conclude that Cassius is trying to say that Brutus is nearly won over. So the “three parts” is probably “three-fourths.”
There is heavy irony here, as Brutus will actually bring about the destruction of the conspiracy.