You don't want to beat your body up too much right before a race. Generally, the advice is to taper your training 1-2 weeks before. However, if you were a runner before your 2-3 month break, then a 5k might not be too much for your body to handle, and you could just train up to a few days before.
Your training should depend partially on your goals (speed? just finishing?), and partially on what you can do now. How far are you able to run now without stopping? If you could do a 5k before your 2-3 months off, you might still be able to do it with a little bit of a struggle. If not, you could try something like:
Day 1: Run 1 mile
Day 2: Run 3 miles
Day 4: Run 2 miles
Day 5: Run 3 miles
Day 7: Run 2 miles
Day 8: Run 3 miles
Day 10: 5k
That will give you a feel for what a 5k is like, since you'll have done 3 miles a few times. But you also won't be pushing it too much before the race. Another approach you could take is to go out on Day 1 and just try to run a 5k to see if you can do it already. If you can, then you could just run 3 miles every other day up until Day 7 or so, and incorporate some interval training to increase speed.
In general, if you can already do the 5k you could concentrate more on your time. If not, you need to build up to where you can complete the distance but not worry about the speed so much.