I wouldn't try to do it myself, unless I had a ton of time on my hands. Well... maybe I would - depending on how bad it was and how much time I wanted to expend on it.
You're going to need to completely empty and/or replace the gas tank - you don't want three year old crud cycling to your engine. If it were me - I'd remove it completely and have it cleaned and checked for cracks before you put gas in it.
Hoses and seals will more than likely all need to be replaced. Major tuneup on the engine. Check all wiring. I've seen boats that sat get a animal in them that chewed up stuff. Trolling motor / sonar - last. Start with the important stuff - running lights, bilge pump, horn - the stuff you will need if you get out there and something goes wrong. Then look into the electronics / trolling motor if they are on there. I'm 100% sure you are going to need new batteries if it has sat for 3 years. If there is any juice left in the ones on there now, they won't last long - I'm sure of that.
I like truggin's suggestion of buying a paddle, just in case your stuck up a creek. :lol:
All the best,
Glenn