Is the SolaX X1-IES-3K compatible with the SolaX X1-Matebox?

Yes — listed by the manufacturer.

The manufacturer's published pairing data lists these together.

Minute Solar records SolaX X1-IES-3K with SolaX X1-Matebox 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.