I'd agree -- if you actually lost 3qt in 800mi - either the pan or the block is ventilated (e.g.; you'd readily see it leaking). The only other place it could go is past the piston rings as blow-by -- and at that rate, you would be able to see blueish smoke out of the tailpipe at idle, never mind accelleration. You'd probably smell it, too, tbh.
I'd definitely grab a sample of what you drained, if you have it.
www.blackstonelabs.com will send you a free test kit via USPS, if you need one. But keep that oil in a safe place, till you get the kit.
If you don't have the old oil, run the new for 500-1000mi (longer is better). You can drain the oil, grab the sample, and reuse it, if you like.
Also... if you still have the old filter, cut it open at the seam (like slicing bologna... best analogy I can think of at the moment), remove the media, and unroll the pleats. Check for any foreign substances (metal, etc.) If you find any, clean them off with some brake cleaner and post back here. Meanwhile, we'll keep our fingers crossed that all is well with your engine. And do yourself a favor and check the oil more frequently for a bit, to see if that consumption rate continues.