Let's say I start my activation with all my defense tokens already exhausted, and then move my ship within range 3 of Suppressor. Does it get to discard one of my tokens? Or is its effect nullified as all my tokens are already exhausted, and it can't "spend" my tokens? The RRG doesn't talk about exhausting tokens, only spending them....though it does mention that spending them the first time flips them to their exhausted side...

for reference:

Suppressor: "After an enemy ship ends its activation, if it is at distance 1-3, you may choose and exhaust 1 of its defense tokens."

You are not spending the token so nothing would happen to it. The same way Overload Pulse works.

Correct. You are Exhausting a Token, which moves it to its exhausted time.

It does specify that an Exhausted Token or Card cannot be Exhausted again.