Let’s assume I have three clock metal routing nets, and I don’t have space in the layout for shielding those three nets, with two of them having the same clock direction. How should I arrange all three, and why?

In this situation, where you have three clock metal routings and two have the same clock direction, you should place the two clock nets with the same direction next to each other, and the third clock net with the opposite direction on the outer side.

Place the two same-direction clock nets adjacent to each other.

Place the third, opposite-direction clock net farther away or on the outer side.

Reason:

When two clock nets have the same direction, the chances of coupling-induced noise (crosstalk) are lower because their signal transitions align more closely, reducing interference. On the other hand, the clock net with the opposite direction has higher potential for coupling noise due to opposite switching, so placing it farther away reduces the impact of crosstalk and ensures better signal integrity.