If you need to do it once:

git branch --no-merged branch1; git branch --no-merged branch2|awk 'a[$0]++==2'

You should count how many branches command you write, and adjust the 2 in the awk line.

If you have to do this operation often, you can write a little shell loop to accept the branch names, and do the same with a dynamic "2" in awk.

Solution

I ended up with a solution similar to the following:

git branch --remotes --no-merged origin/master | \
grep --fixed-strings --invert-match --file=<( \
    echo prod branch2 branch3 branchN | \
    tr ' ' '\n' | \
    xargs --replace \
    git branch --remotes --merged origin/{} \
)

Explanation

Below is the explanation of what different parts of the command above do and their output:

$ echo prod branch2 branch3 branchN
prod branch2 branch3 branchN

- simply outputs names of given branches separated by spaces.

$ echo prod branch2 branch3 branchN | \
>     tr ' ' '\n'
prod
branch2
branch3
branchN

- outputs the same but each branch on a new line.

echo prod branch2 branch3 branchN | \
>     tr ' ' '\n' | \
>     xargs --replace \
>     git branch --remotes --merged origin/{}

- will call the git branch command for each of listed branches, like this:

git branch --remotes --merged origin/prod;
git branch --remotes --merged origin/branch2;
git branch --remotes --merged origin/branch3;
git branch --remotes --merged origin/branchN;

- that is, it'll list all remote branches that are merged into given ones (separated by new lines), e.g.:

  origin/some-branch-merged-into-prod
  origin/branch2A-merged-into-branch2
  origin/branch2B-merged-into-branch2
  origin/some-other-branch-merged-into-branchN

git branch --remotes --no-merged origin/master

- this'll list all branches that aren't merged into origin/master, e.g.:

  origin/some-branch-not-merged-into-any-given-branch
  origin/branch2A-merged-into-branch2
  origin/some-other-branch-merged-into-branchN

Finally, the entire command will exclude the list of branches merged into any of branches given in the echo command from the list of branches not merged into master - thus giving us the list of branches not merged into any of given branches, e.g.:

  origin/some-branch-not-merged-into-any-given-branch

Question already asked I guess

How can I know if a branch has been already merged into master?

if you want to aggregate branches, just :

git branch --no-merged origin/master > file
git branch --no-merged origin/xxx >> file
git branch --no-merged origin/yyy >> file

and count the one that appears 3 times...