Hey Tyc,
Remember in a San He combination, the “middle god” is the birth phase of the combination -> Wu, Zi, Mao, You.
The middle god will directly combine with its tomb if no other same elements are available.
In this chart, You is the middle god, so it directly combines with its tomb Chou.
Had there been another metal element in the chart, then they would enter tomb.
If you replace You with Shen, Shen enters the tomb.
Ren