The Chāndogya Upaniṣad (6.2.3,4; 6.3.3) mentions that Brahman (called ‘Sat’ here) created three fundamental elements out of himself. They are: tejas (fire), āpa (water) and anna (earth).
These three are the pure elements imperceptible to the senses.
Brahman then created the gross elements out of these, by the process of trivṛtkaraṇa or triplication. In this, half of one subtle or sūkṣma element is combined with one-fourth of the other two, resulting in the gross (sthūla) element as follows:
(subtle) tejas ½ + (subtle) āpa ¼ + (subtle) anna ¼ = gross or sthūla tejas.
This is technically called trivṛtkaraṇa.
The process is similar to that of pañcīkaraṇa.