The potting mix (is there garden soil in it?) for the first two begonia pictures above looks very dry. It's harder to tell with the New Guinea impatiens. What is the potting mix comprised of? New Guinea impatiens don't like to get dry but if the stems are going black perhaps there's some stem rot going on. Does that pot have drainage? Does the potting mix feel damp or dry when you push your finger into it? Which direction does that window face?
