Second one looks like buttercup to me, and the third one, the leaves with the square ends is oxalis. Both have nice flowers, but will seed and spread everywhere if allowed.
For future reference, Joel, it's a lot easier to identify a plant that's blooming.
As you say, the term 'weed' is subject to each individual's interpretation. Generally a weed is just any plant growing where you don't want it. Or uninvited. Or too big . . . or??
