How to grep 2 or 3 lines, one containing the text I want, and the others just below it?

Just do a:

grep -A1 ERROR

The -A1 tells grep to include 1 line after the match. -B includes lines before the match, in case you need that too.

For a more portable way, there's awk

awk '/ERROR/{n=NR+1} n>=NR' foo.log

Or maybe you want all the indented lines following?

awk '/^[^[:blank:]]/{p=0} /ERROR/{p=1} p' foo.log