I need assistance to set up a formula to match values between 3 drop down list.

I have 1 drop down named it as ITEMS with 2 listing. HDD and RAM.

The 2nd drop down named it as Size with 2 listing..4 GB and 8 GB.

The 3rd drop down names it as Cap with 2 listing..100 GB and 500 GB.

Who do i write the expression to show that when i choose HDD it should only shown Cap with 2 listing?

Highly appreciate your assistance.