Does this definition (3rd entry) of the word "commission" :
The thing to be done as agent for another.
The thing to be done in place of another thing .
The definition referenced in the question is missing the example sentence that would have put it into better context:
I have three commissions for the city.
It's not something done "in place of another thing."
In this sense, a commission is a task (normally for some kind of finished product) that you complete for somebody else.
Another example that uses this sense might be something like this:
The painting I'm working on is a commission that I have for my client. When I have finished it to her satisfaction, I will give it to her and receive the balance of my payment.
Merriam-Webster provides a more specific definition of this sense of commission:
2 b : a formal request to produce something (especially an artistic work) in exchange for payment
// … he found his footing as a Western artist when he received a commission in 1958 to create two … paintings …
— William Grimes
Actually, there is another specific definition of commission from Merriam-Webster that uses the word agent:
3 b : a task or matter entrusted to one as an agent for another
// executed a commission for me while he was in Singapore
This definition more closely matches that from Wiktionary—however, it's not as close a match to the example sentence it provides.
If the example sentence were meant to be taken in that sense, I would say it would be better phrased in one of the following ways:
I have three commissions from the city.
I have three commissions on behalf of the city
I executed three commissions for the city.
The specific sense meant by the original example sentence is actually somewhat unclear without further context.
However, in this sense, it's not an action that's done in place of another action, but an action that's done by somebody other than the person who was originally supposed to do it. It could be thought of as a task that's been delegated to somebody else.