How to fix the sabotaged medi-gel dispensers?

All you have to do in order to fix them is to click on the reticule, like interacting with any other object. They are located in the following spots. You can fix them in any order:

First - On the wall between Cargo Hold A and B

Second - On the left wall of Cargo Hold A

Third - The north wall at the end of the hall of Bay E28

Just interact with all three of them and return to the doctor, you don't need anything specific for it. It seems Shepard is an expert in any skill needed to save the Galaxy.

If you can't talk to the doctor, you probably missed one of the medigel dispensers.

I've marked the location of the dispensers on the map, they're all in the rear part of the Docks:

The dispensers are scattered around the holding area level. They look like the usual dispensers, you just have to interact with the to fix them. IIRC, one of them should be near the doctor, one near the Turians and one at the end of the holding area.