Dzitmoidonc - YES! I found a photo and it matches perfectly. Whew! This morning I looked at it again and convinced myself it was a poison ivy or poison oak or some kind of poison something. And the squirrels in our neighborhood are NUTS (ha!) for those big walnuts, they drag them around all over the place. Mystery solved. Yay! Thanks everybody!

I agree about the second one being ajuga. I have several in my yard, and they are pretty common perennials/groundcovers, though mine have darker "chocolate" leaves -- it comes in several shades. I don't recognize the first one as any perrenial or flowering plant that I know, and it looks like a bush seedling or sapling to me, saltcedar's idea sounds likely.

The first one looks like eastern daisy fleabane, Erigeron annuus. It's a weed. Here is a link that might be useful: Rob's Plants

The first two are definitely a weed, though I don't know its name. I have done the same as you, let that plant grow into the large sprawling thing you have there in hopes that it was some sort of volunteer beauty, but no, it's not. It has flowers, but they are insignificant, and the plant, while not as unattractive as some, is just not anything you want in your flower bed. The good news is that it is not a particularly noxious weed. At least around here (Southern CA) it doesn't reproduce that wildly. Just pull it out. The third photo looks like some kind of wild geranium; that is, a true geranium, not a pelargonium. At least around here, those are quite nice to have.