I picked fresh oregano, rosemary and purple basil. I placed them in a dark room with a oil heater set to high and a oscillating fan blowing is in the room as well. The heat was on for 4 days, then turned off. The herbs stayed in the room to dry for 3 additional weeks. They all smelled wonderful at this time. Then I place them into air-tight plastic freezer bags and placed the bags into a tightly closed mason jars kept in a closet. After a few days I open the bags and all three herbs smelled bad as if moister remained in the herbs. What did I do wrong? should I have left them out to dry for a few more weeks for a total of 7 weeks? The have been removed from the bags and jars and I placed them into paper bags. But now the herbs don't have any smell.
If they were not dry, and they smelled bad when checked, they are not going to get better - toss them, do better next time.
It's hard to know how an uncontrolled environment drying will work out unless you have experience using that particular system in similar weather patterns - it's aways advisable to pop product you think is dry into glass jars and recheck several times within a day or so (not longer, you want to catch this as soon as it becomes apparent) for any condensation inside the jar, which is a sure sign more drying is needed. If you unjar and continue drying as soon as you spot condensation, it generally won't spoil. You can use bags once you are sure it's dry (though if you are going to put them in mason jars anyway, the bags are a waste of plastic.)
In my personal experience, the basil is better made into pesto and frozen than dried. But you can dry it if that's what you want.
The oregano and rosemary should dry fine. Though if you have any ability to not kill plants, a potted rosemary is not a very fussy houseplant, and you can have it fresh that way. Added after remembering to water mine...
When I had an airy attic I could string herbs up in that got nice and hot, I'd put them there. When I had an oven with a standing pilot light, I'd use that. These days I use a plug-in electric dehydrator with a thermostat and a fan. That gets most things done in a matter of days.
Unless you are in a terribly dry climate I would not regard 4 days of heat and 3 weeks of room temperature (which may not dry at all, depending on the weather) as a particularly good regime - it certainly would not do much in my humid climate. At minimum, some additional time with heat before sealing up would likely help.
There are various other "tests" for dryness of variable usefulness or reliability - do stems snap .vs. bend? - Do leaves crumble .vs. crumple?
If you happen to own or can borrow a dehumidifier, that would be a better way to heat and dry the room you are drying in than a resistance heater (does a better job making it dry, not just warm.)