AT&T is blowing smoke about your router. There's nothing keeping your router from using all 4.5 Mbps of your connection. Or 10 Mbps, or 100 Mbps, or 1000 Mbps. There are other people using PfSense in much larger applications than yours.
But it's worth noting that the speed you're getting on the WAN link to the first router and the speed you're measuring on speedtest.net are not the same. It's pretty much true that only AT&T can measure the connection speed of the T1, unless you somehow have access to the T1 termination equipment and you know how to ask it what the connection speed is. Also, you will lose about 10% or so of the actual connection speed to TCP overhead, and even if you had a Speedtest.net server connected directly to the router at AT&T, the connection speed and "internet" speed will be different as a result of that. So I would expect a maximum capacity of about 4.09 Mbps on 3 T1s and 2.73 Mbps on 2 T1s. If you're getting somewhere in between those two numbers, then AT&T isn't lying to you about the connection speed (especially if you're getting slightly more than 3Mbps - that would be impossible if only two T1s were working).