It depends. If you are planning on using the 3CX multi-tenant installation, then unfortunately, each tenant makes use of a different set of ports. That would make it difficult for multiple companies to share the same gateway. If this will be one instance of the 3CX server hosting more than one company, each with their own PSTN number, then that could be done, possibly awkward, but possible.
You would probably want each customer to have their own trunk group for incoming and outgoing calls, so finding a gateway that doesn't force you to configure all PSTN lines into one group. would be needed.
Look for a gateway that allowed the trunks to be broken into smaller groups, or accessed individually. Some gateways allow this through assignment of a different port to each PSTN line. Some make use of a unique prefix to specify which PSTN line to use.