What IS mandated is that if there is an overload relay, you are required to have OL sensing in all 3 phases, not just two. It used to be allowed to only sense two phases out of three, that's why you still find legacy starters that only have two heaters, but that changed in the code in the 1970s.
If your motor has built-in thermal protection, then you don't have to worry about the OL protection.
But...
2 pole SSRs for 3 phase motors is a really bad idea. In an SSR, if the SCR inside fails, it shorts, meaning it becomes a conductor. If you have 3 pairs of back-to-back SCRs, as is the case with good soft starters, if one SCR fails, no problem because when you turn off the other 5, there is no path for current flow because it is blocked by the other SCRs. The only dangerous condition is if 2 SCRs short in opposite phases, and that is very rare because the voltage or current events that can take out one SCR are usually only on one phase at a time. But with only two pairs of SCRs, one phase is ALWAYS hot, so if just ONE SCR shorts, you cannot turn off the motor unless someone notices and opens the disconnect. When the remaining good SCR pair turns off, the motor is then single phasing and even if you have an OL relay that can sense the single phasing condition, there is nothing it can do about it because you cannot stop the current flow. The result is a burned motor.
By the way, the same is true in your HVAC systems with 2 pole contactors. If one pole welds, the motor fries. It's a really bad idea from the end user standpoint, but in the HVAC world, they go for cheapest way out that isn't technically illegal.
