तेन त्रिवर्णद्विवर्णपुत्रविभागः क्षत्रियवैश्ययोर्व्याख्यातः ॥ कZ_०३.६.१८ ॥
tena trivarṇadvivarṇaputravibhāgaḥ kṣatriyavaiśyayorvyākhyātaḥ
By that same rule, the division among sons in three-varṇa and two-varṇa combinations is explained for the kṣatriya and vaiśya cases as well.
Reuse the established proportional logic (from the four-category formula) to compute shares when only three or two categories are present.