Wikipedia's article on Covenant theology says the following:
Covenant of redemption is a covenant amongst the Godhead in which Christ was appointed to redeem mankind.
Covenant of works is a covenant between God and Adam (representing all mankind) that obedience would bring life and disobedience would bring death.
Covenant of grace "promises eternal life for all people who receive forgiveness of sin through Christ."
Through the covenant of grace, Christ fulfills the covenant of works on behalf of those who receive forgiveness. In this manner Christ also fulfills the covenant of redemption in which he was appointed to redeem mankind.
In my opinion they're not a variation on a theme. They all share the same purpose of salvation for mankind, but they have distinct agreements and expectations of those involved. The covenant of works and covenant of grace are also closely related in that they both promise eternal life, but the former involves God while the latter involves Christ, and the former requires works while the latter requires faith.