First, some general rules:
2 factors in a program are intensity and volume (time or miles spent running). Increase only 1 at a time. If youre going to increase from 3 days of running to 4-6, make those days the same intensity as the others for a couple weeks. That way your body adjusts to the increase in VOLUME first.
Next, when you have your volume (we call this building a base) up, you can change one of your days to increase its INTENSITY. I would start with 1 workout day. Make sure you warm up at least (preferably cool down too). You can do interval training. Or you can do a tempo run. It depends on your goals.
It would be safe to also choose a different day (not the day after or before your interval day) to cal your long run. Gradually increase this by a mile or two each week.
This is how you start.