with galvanic corrosion, you'll want to pick contacting materials that are close together on a galvanic chart like this one http://www.corrosionist.com/Corros1.gif you're likely to see problems in the Aluminum more because of the stainless screws than the brass.

 

You could anodize the aluminum, that will help.
How much stress is the joint under?
You could use plastic or aluminum screws if the joint is not under much stress.
Or stainless with sealing washers under the heads to prevent moisture from getting into the threads and accelerating galvanic corrosion

 

The stainless screw bolted in Al is not a good idea. Electrolytic reaction has been alrdy discussed, but material hardness is also a parameter you'd have to look especially in the screw thread.

If you can't use inserts in the Al plate, it would be better to drill a hole trough the entire material and use a nut or a weldnut made of stainless steal.

regards,

 

Yep, threaded inserts in the Aluminum is also recommended and can be selected to reduce galvanic action. You will still want to isolate the head of the screw from the Aluminum as well in harsh environments.

 

M3 eh? that's tiny. Misumi has em. http://us.misumi-ec.com/us/ItemDetail/10300235320.html
Threaded inserts can be obtained with all sorts of coatings. If your screws are M3 then absolutely you should insert the Aluminum if Aluminum is what you go with. http://www.emhartamericas.com/brands/heli-coil/screw-thread-inserts/materials-coatings-platings

 

LOL because of this thread I had a dream last night that the machine I am currently working on had to be placed outside in the wet and sun. I was freaking out!