Aha. A 3" cut is a long one to do with a Dremel. If you're just doing a few, and if you're patient, no problem. But if you have to do a lot of them, you may find that method to be too slow and tedious for such cuts. For jobs like that, I use my die grinder and bigger wheels.
Or, if the cut is straight, I'll cut part way through with the Dremel, put the blade in a vise, and whack off the waste part at the cut line, using a big chisel to break it. Then I grind the edge smooth with a bench grinder.
If you haven't used those little SiC Dremel wheels, please observe this caution: The wheels shatter easily, if you twist the Dremel out of line even slightly. WEAR EYE PROTECTION! Those little bits can do serious damage to your eyes.
But for short cuts, they're awfully handy. I use those little wheels a lot. They're pretty cheap and very effective.
Good luck!