Extra mushrooms - what to do?
It's perfectly OK to freeze them raw if you're going to make duxelles, which I heartily third.
When you get back, chop them semi-coarsely (I do it in a wood chopping bowl w/a hand-chopper; the processor is too messy) and add some chopped shallots or scallions or onions, and a handful of reconstituted/chopped dried wild mushrooms, if you can, for depth of flavor.
Heat some olive oil/oil & butter/just butter, pref. clarified, in a big skillet, dump in the mushroom/onion mixture and cook,stirring, until it releases its liquid, then turn up the heat a bit and reduce it, stirring frequently, until it's the consistency of a tapenade. Salt, grind in some pepper if you like, add some chopped parsley and a maybe a splash of sherry, cool and freeze flat in 1 qt. freezer bags (burping out as much air as you can) so you can just bang a frozen bag on the counter and break off whatever you need for an omelette/soup/braised dish.
Three lbs. is exactly the amt. I do up in a batch. Duxelles are on the very short list of the best and most versatile flavor-enhancers you can have on hand.
There's no thin line to walk in how finely you chop it up or how much you reduce it. Suit yourself depending on what you most like to use it in.