Best voltage range for a 3.0 ohm carto is 3.6 volts to 5.0 volts.
Please read the article in the below link. It explains about matching voltage to the atomizer that you use. About mid-way down the article you will see a colored chart, called the
Safe vaping Power Chart. This is used by most experienced vapors as a general guideline for effective matching of voltage vs resistance (ohm rating).
Guide to Safe Vaping - E-Cigarette Safety | Ecig Advanced Community Blog
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So if you follow the chart, 6 volts is too high of a voltage for a 3.0 ohm cartomizer (in the red zone).
vaping at 6 volts on a 3.0 ohm carto will burn your juice, causing the burnt taste you describe. If you must go higher than 5 volts, you will need to purchase a 4 ohm cartomizer.
http://www.sweetspotvapors.com/category_s/1828.htm
The crackling sound coming from your cartomizer is normal, although not all cartos will make the sound. The sound is produced when the e-liquid is cooked by the atomizer, much like bacon frying in a pan.
If the juice that you are vaping is mostly PG, it will not produce a lot of vapor. More VG (vegetable glycerin) produces more vapor. You may need to purchase e-liquids that have more Vg in the ratio, or consider 100% VG, if that is more your vaping preference.
I hope you find this info helpful.
