I see many possibilities in Tvastr origins:
1- His role as father of Vrtra, Saranyu and Tris^iras seems to me very similar to Loki in Scandinavia, who was also father of a dragon (Jormungandr), a "terrible" woman (Hel) and to Greek clan of "monster", including dragons (Ekhidna, Typhon, Hydra), an equine goddess (Medousa), a three-headed (Geryon), and a divine horse (Pegasos cf. a horse created by Tvastr and Sleipnir son of Loki). Tvastr-Loki fought Indra-Thor. I'm trying to studying a possible trio of primordial gods slaying the Primordial Man (cf. Odinn-Hoenir-Loki/Odinn-Vili-Ve, Zeus-Hades-Poseidon, Zeus-Kyklops-Hekatongkheir)
2- A common trait of all Smith Gods in Eastern Asia and Mediterranean, like Ugaritic Ktr-Hss,
Sumerian Gibil, Hephaistos, Vulcanus, etc.
3- A Creator role similar to Egyptian Ptah, who also was a "maker" of forms and creatures".
JS Lopes