jpf - first thing I would ask myself is are the large colonies building wax right now? If so, you might get away with borrowing a frame to boost up a nuc' in time for Winter. If not, I'd consider combining the nuc' with the weaker hive.
The nucleus colony is likely too small to effectively build wax for Winter, whereas the larger colonies might be able to do so if there is a good nectar flow going on.
My goal in Winter preparation is to get a 2-deep brood box colony with one super full of honey up to 130+ pounds. If neither of your large colonies are there yet, drop the nucleus idea for this Winter. I wouldn't even consider NOT weighing them on a scale. Just do it.
Try to figure how much more honey and pollen will come in between now and when your area starts getting COLD (ask other long-time beek's in your area their opinions, add these together, and divide by the number of beek's).
Out here on the Left Coast, my Winter usually starts in late-October or November, with a "false Spring" (warm East winds in the dead of Winter) in mid- to late-December. Adjust your estimates according to your late flows in your area.
Feed them high-quality pollen substitutes and liquid feed from now on in to Winter.
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One other approach you might consider is to make a double-screen board and place the nucleus colony over the strongest colony in order to take advantage of the rising heat. Either make an adapter board to the 5-framer to place over the double screen board, or place the nucleus colony in a full-size box.