help please: apply REGEX to multiple lines?

hi friends! happy new year!

i never manage to do a regex includeing multiple lines, does anyone has an idea about that?

imagin you have a file

a

b

c

and you want to turn it into just

b

but without useing grep next line/line before

it needs to be something like:

sed -i "s#a\n(b\n)c\n#$1#g" file (which does not work)

thanks a lot