Yes, some Meraki MX's support 3 WAN uplinks but with some limitations.
Mainly:
- Only certain hardware supports the 3rd WAN port - MX75, MX85, MX95, MX105
- The feature is only available from firmware 18.2 and higher
- The third WAN port can only be used as a backup link
Additional information can be found in the link below.
MultiWAN Backup Uplink - Cisco Meraki Documentation