I SECOND, THIRD AND FOURTH THAT! Both for the Slime and ride.

I remember the day I finally rode 10 miles...I could not imagine at some point I would routinely ride 100+ miles in a day!

As to equipment, everyone has good advice. The best is knowing how to fix a flat and what it takes to do that. The tires on your bike are probably heavy duty (emphasis on the heavy) as it is - more important is making sure the tire pressure is correct. Nothing causes flats more than improper tire pressure.

The good thing about only riding 3 miles is that means you only a short distance to walk back. I would not get too crazy about loading yourself down with stuff now. A spare tube, tire irons and a pump will probably do you for now. As you extend your distance you can add in more equipment or accessories. If you are comfortable patching tires, carry a patch kit. I don't anymore since I don't trust patched tires but I have friends whose tubes are totally pink from so many patches.

Remember you and your bike right now are heavy... heavy is tough to push around. As you ride your body will lightened up and as you ride, you hopefully will upgrade your bike to be lighter as well. The key now is to just keep moving... 1 mile or 3 or 5 doesn't matter... just keep at it!!!