Is a multi script possible that requires relative time locks relative to each signature?
Your question is a nice thought experiment. 

Your example would not itself work: as CHECKSEQUENCEVERIFY is relative to the mining date or height, so in week 3, all three parties could sign at the same time.  On day 1 things work as you intend. On day 8 'X' can now sign at any point without delay but must wait a week for Y and two for Z. On day 15, 'X' and 'Y' can both sign at any point without delay but must wait a further week for Z. On day 22 all three of them can sign without delay, and it's no different to a regular multisig.

The direction of your thoughts seems to be that CHECKSEQUENCEVERIFY could be made relative to a previous signature. I'm not aware of any method of doing this within a single script: how can a computer tell that a signature has been made more than a week prior, other than by seeing it on the blockchain? The mempool would be wholly inappropriate a venue not least because people would be storing something for potentially long periods of time.

An improvement of sorts would be to have the multisig lock 10% of the funds for ten months, and each month, lock another 10%. At the start of the eleventh month, the three of them redeem to a new multisig, and so relock that original 10% for a new set of ten months. In this manner, all three of them are needed and 90% of the funds are locked for at least one month even if they're all present.