I am ready to do a brisket. The tools I have are:

1. Electric smoker
2. Gas grill
3. Oven

Given those 3 tools, how would you recommend cooking a brisket?

I was considering trimming the fat, injecting with butter or beef stock, homemade rub, smoke for 3 hours around 225°, wrap in foil and finish off in the oven, rest in cooler for 1 hour or so.

Gotta trim that deckle fat off, it will not render down. They only reason I would inject a brisket is if I had a select graded brisket. To me 3 hours is not long enough at that low temp to get you a nearly decent bark, also the meat will still be at a temp where it will take on a smoke flavor. I think its 160-165 IT before meat quits taking on the smoke flavor.

Probably isn't a perfect way to do it.

Salt and pepper is all it needs on the outside. A little trimming wont hurt but don't get too carried away brisket isn't suppose to be lean or heart healthy. I don't know weather it will take more smoke or not after 3 hours, but 3 hours of smoke will leave you with a fine end result have done it before. While not the traditional way to cook bbq, finishing in the oven is a much easier way if you have a smoker your have to work at to maintain a set temp.

electric smoker for sure.

I did your plan a while back, OP, and it made the house smell too much like smoke. Just wrap it and put it on the grill or back in the electric smoker at moderate heat once your 3 hours is done.