In plain words
Among weapons I am the thunderbolt; among cows I am the wish-fulfilling cow; I am Kandarpa, the power of procreation; among serpents I am Vasuki.
Word for word
Among weapons I [am] the thunderbolt; among cows I am the wish-yielding [cow]; and I am Kandarpa the begetter; among serpents I am Vāsuki.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
आयुधानाम् अहं वज्रं दधीच्यस्थिसम्भवम् । धेनूनां दोग्ध्रीणाम् अस्मि कामधुक् वसिष्ठस्य सर्वकामानां दोग्ध्री, सामान्या वा कामधुक् । प्रजनः प्रजनयिता अस्मि कन्दर्पः कामः सर्पाणां सर्पभेदानाम् अस्मि वासुकिः सर्पराजः ॥ २८ ॥
In plain words
"Among weapons I am the thunderbolt, the one that came out of Dadhīci's bones." "Among cows, among the ones that actually give milk, I am Kāmadhuk" — Vasiṣṭha's cow, who yields every wish; or simply the wish-yielding cow, taken generally. "Prajanaḥ" — the begetter — "that I am; Kandarpa" — desire. "Among serpents" — meaning among the kinds of serpent — "I am Vāsuki": their king.
Close to the Sanskrit
"Of weapons I am the thunderbolt, arisen from Dadhīci's bones." "Of cows, of those that yield milk, I am Kāmadhuk" — the yielder of all desires belonging to Vasiṣṭha; or, kāmadhuk in the general sense. "Prajanaḥ" — the begetter — "I am; Kandarpa" — desire. "Of serpents" — of the kinds of serpents — "I am Vāsuki" — the king of serpents.