द्यौरेवर्गादित्य साम तदेतदेतस्यामृ-च्यध्यूढं साम तस्मादृच्यध्यूढं सामगीयते द्यौरेव सादित्योऽमस्तत्साम ॥ ३ ॥
In plain words
The sky itself is the ṛk and the sun is the sāman. This sāman rests upon that very ṛk, which is why the sāman is sung as resting upon the ṛk. The sky is sā and the sun is ama, and together they make sāma.
Close to the Sanskrit
The sky indeed is the ṛk, the sun is the sāman. This sāman is founded upon this ṛk. Therefore the sāman is sung as founded upon the ṛk. The sky indeed is sā, and the sun is ama, and that is sāma.