A single sed line with multiple commands should do the trick:
sed -e 's/&//g' -e 's/\t/\&/' -e 's/$/\\\\/g' file
moreover sed offers the -i option to edit the file in place. Use it with caution, until you're sure of the results or add a suffix to make a backup copy of the original file, e.g.
sed -i.bck -e 's/&//g' -e 's/\t/\&/' -e 's/$/\\\\/g' file
will create a backup copy called file.bck preserving the original content if soemthing goes wrong.
Another option is process substitution
, a shell feature useful to avoid the creation of temporary files. In this case it's easy to use sed as shown, but sometimes it might be handy. For example suppose you have to check the difference between two unsorted files and you want to sort them previously:
sort file1 > file1.sorted
sort file2 > file2.sorted
diff file1.sorted file2.sorted
You can avoid the creation of the sorted files using:
diff <(sort file1) <(sort file2)
Hope this helps.