I see dollarweed and oxalis, both can be killed without killing your existing grass. dollarweed can be killed with any broad weed control like weed b gon, oxalis can be killed with any product containing triclopyr.
Good luck.

Thanks for your reply jdo... Because you menioned dollar weed, I went onto the Clemson U website and discovered that the weed we have is dichondra. Dollar weed is very similar, but after comparing stems, leaves, etc, we determined it is dichondra. Now that we know what it is, we are wondering if anyone has ever tried to eradicate dichondra from their lawns. It looks like we may have to use Roundup, and if that is the case, it will destroy the lawn too. But if that is the only recourse, then that's what we'll do. Anybody ever had this problem?

Thanks.

dichondra is killed by triclopyr, won't harm the existing grass. no need to kill the whole lawn unless you want to.

http://www.turffiles.ncsu.edu/weeds/Dichondra.aspx

Hi jdo... What products have triclopyr?

ortho weed b gon chickweed, clover and oxalis killer is one. Some of the brush and poison ivy killers have triclopyr as well.