You can use . to refer to the current line.
1) :g/foo/.,+2d will delete line with text foo and 2 lines after it
2) :g/foo/-3,.d will delete line with text foo and 3 lines before it
3) :g/foo/-1,+1d will delete line with foo, one line before it and one line after it
1) :g/foo/.,+2d will delete line with text foo and 2 lines after it
2) :g/foo/-3,.d will delete line with text foo and 3 lines before it
3) :g/foo/-1,+1d will delete line with foo, one line before it and one line after it