I believe that's an amaranth, Amaranthus sp. I call them red root pigweed, and yours looks large and healthy.
It did produce a flower (the large 'caterpillar' at the stem apex), which means soon there will be seed, and lots of it. I would recommend getting rid of the plants asap.
If this is the first time you've encountered this plant, controlling it now will stop the chances of it becoming a noxious weed at your place (like it is at mine). If it did come in the 'assorted seeds', you may get more seedlings.
They may not pull very easily, so you might want to dig them out, to get the entire root. At least you didn't get one of the spiny species (which I also have).
It is also edible, as a green and a grain. if you are interested in eating the greens, cut the flowers off so it doesn't reseed the whole garden. You can harvest the younger, more tender leaves and prepare them like spinach.
If you want to try and save some seed for grain (not that practical with only three plants), leave the flower heads until they mature into seed-heads. Then, when the plant starts drying out, tie a paper bag on the seed head and cut the stalk. Hang in a warm, dry place until completely dry. Then you can rub/shake the seeds off the plant, into the paper bag.