Welcome to the dark side John.
For 3 sec exposures all you need to do is program you ISO into the camera which you can find out how to do in the manual, set your exposure time, you can do up to 30 sec using the camera timer and you shouldn't get too much field rotation is you are only using a short FL lens and then get a bulb release cable. And that's it. If you want to automate then yes you need a computer.
With the bulb cable all you need to do is set your exposure time, set the camera to multiple exposure and click and lock the bulb switch down. It will keep taking exposures at your required exposure length until your turn off the switch. Easy as.