In act 3, scene 3, Rivers and Gray, the queen's kinsmen, and Sir Thomas Vaughan, their friend, all die offstage. Their deaths are only confirmed in act 4, scene 4, when Margaret exclaims that they have all been "untimely smother'd in their dusky graves." As they are about to die, in act 3, scene 3, Gray says:

Now Margaret's curse is fall'n upon our heads,
For standing by when Richard stabb'd her son.

He and the others realize that Margaret's curses are coming to pass. Their deaths might be considered as some kind of justice because they could have done more to prevent Richard from killing Margaret's son. However, their deaths might also be considered unjust because they are essentially loyal and dutiful characters, and because their deaths are ordered by Richard for no morally justifiable reason, but only to advance his own plans.

In act 3, scene 4, Richard orders Hastings's execution, ostensibly for treason. We know that Hastings hasn't really committed treason against Richard, and in fact Hastings is genuinely loyal to Richard, admitting to a "tender love." Nonetheless, Richard is unimpressed that Hastings was reluctant to join his plan to murder Prince Edward, so he accuses Hastings of conspiring with, or being a "protector of this damned strumpet," meaning Elizabeth, and, on this hollow pretext, swiftly declares, "Off with his head!"

In terms of the justice or otherwise of Hastings's death, this too can be argued both ways. On the one hand his death was unjust because the reason for it (conspiring with Elizabeth) was patently false. He is innocent of the charge. He is also loyal to the throne of England, as opposed to one faction or another. However, one might also argue that he has been naïve about Richard, and has therefore lent tacit legitimacy to Richard's reign of terror. He is being punished, perhaps, for standing by and doing too little to stop Richard.

Rivers, Gray, Vaughn, and Hastings all die in Act 3 of Richard III. Richard has orchestrated their deaths as part of his master plan to seize the throne. As the previous answer says, they all realize that wretched Queen Margaret's curse on them all has come to pass: they have been killed because of Richard's wicked ways.

As for what this suggests about justice, one thing that is suggested is that on earth there is no perfect justice. Grey and Rivers do observe that they stood by while "Richard stabb'd [Margaret's] son," which does suggest their deaths are in a way a form of retribution. After all, Richard III is sometimes interpreted as being a "scourge of God": that is, a bad or corrupt monarch who is used by God to punish a wayward nation. In Hastings' case, there seems to be a lack of justice altogether.

However, the play ends with justice triumphing. Richard is killed and overthrown by Richmond, who marries Elizabeth of York and becomes Henry VII. Richard, who has killed most of the characters in the play, is killed himself as a result of his evil.

First of all, Rivers, Grey and Vaughan all die together in Act 3, Scene 3. What do they realise as the die? That they're dying because of Richard's whim, and that the curse that Queen Margaret made earlier in the play (which predicted that they'd all meet sticky ends because of Richard) has come true:

GREY
Now Margaret's curse is fall'n upon our heads,
For standing by when Richard stabb'd her son.

RIVERS
Then cursed she Hastings, then cursed she Buckingham,
Then cursed she Richard. O, remember, God
To hear her prayers for them, as now for us
And for my sister and her princely sons,
Be satisfied, dear God, with our true blood,
Which, as thou know'st, unjustly must be spilt.

Ominously, Rivers even seems to know there at the end that there's more blood to come - praying for his sister (Elizabeth) and her sons (the two princes in the tower).

Next scene, next death. Hastings, Act 3, Scene 4. And he realises exactly the same things:

O Margaret, Margaret, now thy heavy curse
Is lighted on poor Hastings' wretched head!

O bloody Richard! miserable England!
I prophesy the fearful'st time to thee
That ever wretched age hath look'd upon.
Come, lead me to the block; bear him my head.
They smile at me that shortly shall be dead.

What does this all suggest about justice? That there isn't any actual justice on earth: Richard is just removing everyone who stands in his way. But - and the play bears this out - that Margaret's curse is going to come true of Richard too.