What are three incidents that could have foiled the conspirators' plans in Act III, Scene 1, of Julius Caesar?

The first incident in this scene relates to Caesar telling the soothsayer, "The ides of March are come," and the soothsayer replying, "Ay Caesar; but not gone." This interaction stems from Act I, Scene 2, during the feast of Lupercal, when the soothsayer warned Caesar to, "Beware the ides of March," which falls on the fifteenth day of the month. Caesar dismissed the soothsayer by saying, "He is a dreamer; let us leave him: pass."

If Caesar had heeded the prognosticator's urgent warning and taken steps to protect himself or remained at home on March 15, the assassination would probably not have happened. Instead, Caesar was so confident in his invincibility and importance that he rejected the soothsayer's alert with a sneer.

The second incident is presented in the form of a similar forewarning by way of a letter Artemidorus wants to give Caesar. In the letter, he wrote the following:

Caesar, beware of Brutus; take heed of Cassius;
come not near Casca; have an eye to Cinna, trust not
Trebonius: mark well Metellus Cimber: Decius Brutus
loves thee not: thou hast wronged Caius Ligarius.
There is but one mind in all these men, and it is
bent against Caesar. If thou beest not immortal,
look about you: security gives way to conspiracy.
The mighty gods defend thee! Thy lover,

'ARTEMIDORUS.'

Artemidorus's script is much more specific than the soothsayer's. Unfortunately, Caesar rejects Artemidorus's request to read his letter first. Instead, Caesar asks him whether he is mad. As a result, Caesar misses another opportunity to prevent his untimely doom. If Caesar had heeded Artemidorus' warning, he could have had those named in the letter placed under close watch or prevented them from approaching him directly, thwarting their plans.

The final incident comes in the form of Popilius, who approaches Cassius and says the following:

I wish your enterprise to-day may thrive.

He is, obviously, referring to the plot to assassinate the general. Cassius informs Brutus of his fear that their plot had been uncovered and believes Popilius will inform Caesar of their malicious plan. He says,

Casca, be sudden, for we fear prevention.
Brutus, what shall be done? If this be known,
Cassius or Caesar never shall turn back,
For I will slay myself.

Brutus, however, assures Cassius that Popilius will not betray their plot, saying,

Cassius, be constant:
Popilius Lena speaks not of our purposes;
For, look, he smiles, and Caesar doth not change.

Popilius, for reasons unknown, does not tell Caesar about the conspirators' plan, allowing the conspirators to execute their pernicious plan unhindered.