What difference does it add when you write the で particle after words like それ and これ?
これで can generally be translated as "with this". It's used to indicate the result or conclusion following some action or happening. For example:
With this, today's activities come to a close.
You can find more of these examples on Jisho's sentence search.
For これは and これが, you need to look up the differences between は and が, which is a complex topic (but it's been answered many times before). Generally, they mean "this is".