Well, that didn't seem to work (and almost seems like something is really not working here - based on the field descriptions and your comments).

Promotion 1 -
Type: Applies to all purchases / Fixed Amount
Priority: 1
Exclusive: No
To which plans this promotion applies: Applies to ALL
How applies: until depleted
Simultaneous purchases required: None
Conditions:
Plan: A
Must be in state: Active
Or can be purchased at same time: Yes
AND
Plan: B
Must be in state: Active
Or can be purchased at same time: Yes

Promotion 2 -
Type: Coupon / Fixed Amount
Priority: 11
Exclusive: Yes
To which plans this promotion applies: ALL
How applies: until depleted
Simultaneous purchases required: None
Conditions:
Plan: A
Must be in state: Active
Or can be purchased at same time: Yes

Promotion 3 -
Type: Coupon / Fixed Amount
Priority: 11
Exclusive: Yes
To which plans this promotion applies: ALL
How applies: until depleted
Simultaneous purchases required: None
Conditions:
Plan: A
Must be in state: Active
Or can be purchased at same time: Yes

Promotion 4 -
Type: Coupon / Percentage
Priority: 11
Exclusive: Yes
To which plans this promotion applies: PLAN A, PLAN B, PLAN C
How applies: until depleted
Simultaneous purchases required: None
Conditions:
Plan: A
Must be in state: Active
Or can be purchased at same time: Yes


When we have Plan A selected, then

* Promotion 1 is automatically applied (good)
* Promotion 2 can be added (good)
* Promotion 3 - can NOT be added if Promotion 2 is already added (good); CAN be added if no other promotions (good)
* Promotion 4 - can be added even if Promotion 2 or 3 have already been added (BAD)

It almost appears that when it is a percentage coupon that the exclusive check isn't being made.

Well, strange. I tried using another discount (Identical to Promotion 4) and it worked properly (got the 'already' promotion notice).
Then went back to Promotion 4 and it gave me the proper notice also.

(Maybe a caching issue?)

Anyway, it LOOKS like all is working now.