will and shall are basically the same thing and mandatory.
"should" is optional.
what else were you looking for?
will and shall mean the same thing. they mean it is mandatory. the use of the which word depends on who wrote the contract.
it wouldn't matter which word was used, the meaning is the same. required/mandatory.
the only difference that confuses some people is the work "should", which indicates a recommendation, but not a requirement.