Is the Fox ESS H3-Smart H3-9.9-Smart compatible with the Fox ESS EPS BOX PRO 63A?

Yes — listed by the manufacturer.

The manufacturer's published pairing data lists these together.

Minute Solar records Fox ESS H3-Smart H3-9.9-Smart with Fox ESS EPS BOX PRO 63A 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.