Satapatha Brahmana II. 1.4.11
“(Uttering), ‘bhuh’, Prajapati generated this earth. (Uttering)‘bhuvah’ he generated the air, and (Uttering) ‘svah’ he generated the sky.This universe is co-extensive with these worlds. (The fire) is placed with the whole. Saying ‘bhuh’, Prajapati generated the Brahman; saying ‘bhuvah’’ he generated the Kshattra; (and saying) ‘svah’, he generated theVis. The fire is placed with the whole. (Saying) ‘bhuh’, Prajapati generatedhimself; (saying) ‘bhuvah’, he generated offspring : saying ‘svah’, he generated animals. This world is so much as self, offspring, and animals. (The fire) is placed with the whole.”
Taittirya Brahmana III. 12.9.2
“This entire (universe) has been created by Brahma. Men say that the Vaishya class was produced from ric verses. They say that the Yajur Veda is the womb from which the Kshattriya was born. The Sama Veda is the source from which the Brahmins sprang. This word the ancients declared to the ancients.”
Why are Brahmanas speaking of only three Varnas? Was there never a Sudra Varna? What's the context of these hymms?