Am i right in saying that "diese = these", "dieses=that", and "dieser=this".
No, you are not.
dieser, diese, dieses before a masculine/feminine/neuter noun translate to "this"
diese before a plural noun translates to "these"
So diese is ambiguous -- for example, diese Leiter could be either feminine singular (this ladder) or it could be plural (these leaders).
"that" or "those" are not usually a good translation for dieser, diese, dieses.