How to configure a Static IP's assigned by AT&T on an ON Demand fiber circuit. In a TZ300

You can only assign 1 address to the WAN port. However, as long as the other public IPs are in the same subnet as the main one, you can create Address Objects and utilize them however you please (be it for port forwarding, NAT rules, etc).

Where does this fiber circuit terminate?
It sounds off. Did the on-demand AT&T provide you two blocks of IPs
WAN a.b.a.c.0/30 as an example
With your wan IP as a.b.c.2/30 and the default gateway as a.b.c.1
They then provided you with a LAN block let's say
LAN d.e.f.0/28 (14 LAN IPS
Commonly the available IPS are d.e.f.2-14 are allocatable with netmask of 255.255.255.240 default gateway of d.e.f.1

In this type of allocation, AT&T would have already configured their router to provide LAN access to systems/devices behind it.
In your case the LAN side of the on-demand fiber has to be connected into a switch to which the thre TZs will connect and that is where you would configure the WAn portion of the TZ with the LAN IP from AT&T

Presumably you are configuring Your TZs with two outside interfaces one for the AT&T fiber and the other for what they had if any....

Isp1.               AT&T on demand fiber
      \.                            /
    TZ port1.              TZ port2
      ===============
                   ||
             NATTED LAN

Any updates?

I just thought about this question more. Did AT&T give you 2 IP ranges? If so, then what you need is a layer 3 device like a router or a layer 3 switch. That will handle the routing aspect for you. Then you can have the 3 TZ300s connected to the device you opt to use. Comcast has similiar type situations with their fiber service.