Spent a lot of years working on 'Beowulf' and I reckon that the monsters represent human characters. In my view: Grendel represents Agnar, son of Ingeld; Grendel's Mum represents the daughter of Earl Swerting of Sweden (and the first wife of Ingeld); and the Dragon represents Onela, king of the Swedes. I think that there has been a scribal error right at the beginning of the poem, which has made Scyld's 'bearn' (Modern English, 'bairn') into Beowulf the First. Thus the real parallels of the poem have been lost.
As far as the first two monsters are concerned: certain Scandinavian tales say that the father of Halfdane is Froda, and that Froda's second son is Ingeld. In one of those tales, Ingeld kills Halfdane and is cast out of the kingdom. In the poem, Grendel is forced to live in the homeland of the monsters after God has condemned 'him' (not 'them', or the monsters, as it is usually said) as being of the kin of Cain.
Grendel's mother suffers after 'Cain' kills his brother Abel with the edges of a sword. This would mean that the Christian element is a crucial part of the poem. Agnar is cut in half by Bothvarr Bjarki, who has been thought by many to be the equivalent of Beowulf. Agnar dies 'with his lips separated into a smile'. This may be reflected in the very name 'Gren-del', which is likely to mean 'Grin-Division' or 'Grin Deal' if looking at it in an older form of the English language. When the fight with Grendel is over, and everybody knows his name (given that he has fought the Danes for years, and has been named before), it is said that 'men in days of yore called him "Grendel"'. I believe that this is a hint for the audience.
The Report to Hygelac tells about the proposed marriage between Ingeld and Freawaru. This episode appears to be a rewrite of the Scandinavian tale in which Ingeld rose up against his first wife's family at the urging of the old warrior, Starkarth.
For the last monster: there is talk of the Swedish king, Ongen-theow, which means 'against-the-slave'. In an earlier tale, Egill fights against the slave, Tunni, who has stolen his treasure. Egil is the father of Ohthere and Onela. So is Ongentheow in our poem. I think that the dragon's hoard represents the treasury of the kingdom of Sweden, or perhaps Geatland.
Onela kills Beowulf's cousin, Heardred, and allows Beowulf to hold the throne, but Beowulf later fights Onela in an expedition and kills him. The poem tells of the expedition at length, and then it is said that 'by then' it was known why the feud had arisen'. It is also said that the Dragon kills 'a certain man' (Heardred?) and that the 'feud is then savagely avenged'. And when Beowulf is at the point of death, he says that he is grateful that, when his soul passes forth from his body, God will not 'censure him for the treacherous murder of kinsmen'.
Onela kills his own nephew, Eanmund, in the same battle (or attack) in which Heardred is killed. Eanmund's sword is brought to Beowulf in his time of greatest need. It has been taken by Weohstan from Eanmund, given to his son Wiglaf, and then brought by Wiglaf to Beowulf when he needs it. He uses it, apparently as an instrument of revenge, to kill the Dragon.
When the Dragon is first awakened by the 'slave' (who might be seen, loosely, to remind readers of Tunni), it is said that 'strife was renewed'. There has been ongoing strife for many years between the Swedish kings and the Geats (under Hygelac and Heardred and Beowulf). This feud appears to be one created after a marriage between Ongentheow and the sister of Hrethel. This woman might be the one who appears at Beowulf's funeral, lamenting that she fears harm for herself in days to come -- which perhaps means that her descendants will be fighting over the various rights to either the Geatish or the Swedish thrones. In the poem, marriages are meant to end political tensions and cement ties between nations, but they very rarely do so, as in the case of Hildeburh.