What comes first

[MisterD]

I have a character with a few compund powers.

A HA/PD drain
a HA/rPD drain
and an EB/ED drain.

My questions is.

When constructing the power can I say what affects the target first or is there a rules saying what affects the target first (Drains or attacks)

[Steve Long]

By "compound" I'm assuming you mean Linked.

The rules on 5E 196 state that if part of a Linked power reduces the target's defenses or CV, that part applies last. In all the situations you describe, the Drain takes place second, after the damaging part of the attack.