Grimoald 3 was the son of Gisulf and Romilda, brother of Taso, Cacco and Raduald and of four sisters, Appa, Gaila and two others whose names are unknown: see PLRE III, Gisulfus 2, p. 537, and stemma 20, p. 1553.

Grimoald 3 was the father of Romuald (Romoald 1): Paul. Diac., Hist. Lang. IV 51, V 7. He also had a daughter, Gisa 1, sister of Romoald 1 (presumably from the same mother): Paul. Diac., Hist. Lang. V 8. He later married a daughter of Charibert 1 (apparently his second wife) and had a son, Garipald 2 (see below). He had three grandsons, Grimoald 4, Gisulf 1 and Arichis 1, by the marriage between Romoald 1 and Theuderada 1 (daughter of the dux of Forum Iulii, Lupus 1) (see below).

Grimoald 3 was the Lombard dux of Beneventum ("Grimuald Beneventanorum strenuum tunc ductorem"); the Lombard king Godepert 1 sent an envoy Garipald 1 to Grimoald 3 seeking Grimoald 3's aid against the other king, Perctarit 1 (Godepert 1's brother), and promising Grimoald 3 the hand of Godepert 1's sister in marriage; Grimoald 3, however, was persuaded to aim for the throne himself, on the grounds that the brothers' quarrels were endangering the kingdom; he is described as mature, wise and strong ("aetate maturus, consilio providus et viribus fortis"); leaving his son Romoald 1 at Beneventum, Grimoald 3 advanced north, securing support in the cities as he went through and winning over Spoletium and Tuscia with the help of Trasimund 2; from Placentia he entered Ticinum, staying in the palace, where, after some devious scheming by Garipald 1, Grimoald 3 assassinated Godepert 1 and took possession of his kingdom; however Grimoald 3 allowed the infant son of Godepert 1, Raginpert 1, to live; the flight of Perctarit 1 gave Grimoald 3 possession of the whole kingdom (and cf. Rodelinda 1): Paul. Diac., Hist. Lang. IV 51, cf. VI 18 (killed Godepert 1). The date was c. 662.

In July 668 Grimoald 3 was in the sixth year of his reign ("anno D. p. sexto regni mei, mense Iulio, indictione undecima"; i.e. July 668): MGH, Leg. IV, p. 91 ("prologus legum"). He therefore ascended the throne in either later 662 or early 663, and probably before Easter 663 when Garipald 1 was probably murdered at Turin.

Soon after Grimoald 3 was proclaimed king at Ticinum, he married the daughter of king Aripert (Charibert 1) and sister of Godepert 1 (Anonyma 48) to whom he had previously been betrothed: Paul. Diac., Hist. Lang. V 1.

Grimoald 3 threatened to end the peace with the Avars if the khagan (Anonymus 308) continued to give shelter to Perctarit 1; the threat was enough to induce the khan to send Perctarit away: Paul. Diac., Hist. Lang. V 2. He defeated an invading army from Francia; he abandoned his camp, leaving lots of food and wine, and then attacked and killed the Franks when they were overcome with sleep and wine: Paul. Diac., Hist. Lang. V 5.

In 663 the emperor Constans II (Konstans 1) invaded southern Italy and laid siege to Beneventum, commanded by Grimoald 3's son, Romoald 1; Grimoald 3, then north of the river Po (in Ticinum), received an envoy, Sesuald 1, from Romoald 1, asking for help and he set out forthwith (he entrusted the care of his palace at Ticinum to Lupus 1: Paul. Diac., Hist. Lang. V 17); en route to Beneventum, a number of Lombards deserted him; however, the news of Grimoald 3's approach alarmed the emperor: Paul. Diac., Hist. Lang. V 7. Konstans 1 then withdrew to Naples: Paul. Diac., Hist. Lang. V 7-9. Grimoald 3 then entered Beneventum, where he celebrated a victory by Romoald 1 over a Greek army under Saburrus 1: Paul. Diac., Hist. Lang. V 10. All this occurred before 5 July 663, when Konstans 1 visited Rome; see Lib. Pont. 78.

Grimoald 3 then returned to Ticinum, having saved Beneventum and the surrounding districts from the Greeks and appointed Trasimund 2 to be dux of Spoletium; he gave one of his daughters, Anonyma 49, in marriage to Trasimund 2: Paul. Diac., Hist. Lang. V 16. He later punished all those who had deserted him on his way to Beneventum: Paul. Diac., Hist. Lang. V 26. He also took his revenge on the Roman city of Forum Populi (Forum Popilii, in Flaminia), which had harassed him while en route to Beneventum and had molested his envoys travelling to and from Beneventum; on Easter Sunday he attacked the city and killed most of the inhabitants, so much so that, according to Paul the Deacon, the city had few inhabitants in his own day: Paul. Diac., Hist. Lang. V 27.

Grimoald 3 is said to have been particularly hostile to the Romans because of their part in the murder of his brothers Taso and Cacco (see Paul. Diac., Hist. Lang. IV 38 and cf. PLRE III, Taso 1, p. 1218) and he therefore destroyed the city of Opitergium where they had been killed and distributed its territories between Forum Iulii, Tarvisium and Ceneta: Paul. Diac., Hist. Lang. V 28. He summoned the khagan of the Avars (Anonymus 308) to crush for him a rebellion at Forum Iulii by Lupus 1; after defeating and killing Lupus 1, the Avars refused to abandon Forum Iulii until Grimoald 3 began assembling an army against them; convinced by him that he had overwhelming numbers, they returned home: Paul. Diac., Hist. Lang. V 18-21.

After the death of Lupus 1, Grimoald 3 married his son Romoald 1 to Lupus 1's daughter, Theuderada 1; from the marriage he had three grandsons, Grimoald 4, Gisulf 1 and Arichis 1: Paul. Diac., Hist. Lang. V 25. He welcomed a group of Bulgars led by Alzeco 1 into Italy and sent them to his son Romoald 1 at Beneventum who settled them in vacant territories around Beneventum: Paul. Diac., Hist. Lang. V 29. He made a peace treaty with the Franks under Dagobert (Dagibert 1): Paul. Diac., Hist. Lang. V 32 (it is questionable if Dagobert II was Frankish king at this time).

Grimoald 3 seized the throne one year and three months after the death of Charibert 1 (Aripert I) and reigned for nine years; he died as a result, allegedly, of poisons applied by his doctors when wounds due to a previous phlebotomy opened and began to bleed; he was buried at Ticinum in the basilica of St Ambrose, which he himself had built; he was survived by his young son, Garipald 2, born to him by the daughter of Charibert 1 (Anonyma 48), who briefly succeeded him; Grimoald 3 is here described as a big man, bold, bald, bearded, and as wise as he was strong: Paul. Diac., Hist. Lang. V 33. He became king in 662 or 663 and died in 671 or 672.

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