for Nach Waxman brisket how long should I cook 3 lbs of meat? L
I've cooked brisket many times, but not adapted the classic Nach Waxman to a 3 lb piece of meat (half the size in his recipe).
That said, here's my cautious advice, being careful to cook but not overcook your meat. If someone else jumps in with advice based on this exact adaptation, I would go with that.
Probably make his recipe as written through the cooking of the (whole) meat about 1.5 hr in oven. (That won't hurt your brisket, as most recipes call for about an hour per pound. To be safe, check meat internal temp at 1 hour and remove if it passes 180F or 82C, as there is more cooking to do. For reference on desired internal temps, see this article with advice from grill masters Chris Schlesinger and John Willoughby.
Then, back to Waxman recipe, slice as he recommends and return to oven.
This point is where I think your cooking will diverge...and you will possibly/probably need less than the whole second roasting of 1.5-2 hours.
Again, check the internal temp at intervals (maybe leave one slice largish, to accommodate the meat thermometer) and remove the meat from the oven when it is fork-tender, as he says, and/or reaches 190-205F (again, see linked article for ranges and what you know you like).
I hope it turns out well...