Is the SolaX X3-FIT W X3-FIT-15.0-W compatible with the SolaX X3-Matebox Basic?

Yes — listed by the manufacturer.

The manufacturer's published pairing data lists these together.

Minute Solar records SolaX X3-FIT W X3-FIT-15.0-W with SolaX X3-Matebox Basic as compatible (A — Automatic). Derived from recorded specifications or compatibility groups; no manufacturer approval is recorded.

▦Manufacturer's compatibility listListed together in the manufacturer's published pairing data. A cited document reference is being added as datasheets are processed.