Depends on what "relays" you saw. But in general, you can calculate current on each of all three phases from measuring two, because from a vector standpoint, the numbers must net to be zero, meaning all of the current going out of one phase must come back on the other two. So if you are looking at primary current on 2 out of 3 of the phases, the secondaries, if tied to the meter in the same configuration as the primary, will match. So current on the secondary of the phase you don't measure has to be the remainder. So yes, you do it to save the cost of that 3rd CT and mounting, wiring etc.
The one caveat is if any of the current from any phase goes to ground, the vector sum will not be zero, but if that happens on the phase you are not actually measuring, you won't know it, which is what a ground fault relay is looking for. so to provide Ground Fault Protection you must look at current in all three phases. One method of providing GFP is to pass all of the conductors through one core balanced CT called a Zero Sequence CT (ZSCT), so that if the three phases (and neutral) don't net out as zero, a current is generated on the secondary of that CT and is used to trip the protection relay. The other method is called Residual Sensing (RS) or Residual Current Detection (RCD) that looks at the specific current going through separate CTs and calculates that residual. RCD is less sensitive, meaning the residual current needs to be a significant value to avoid nuisance tripping from noise on the system, so if you have a resistance grounded system where you are specifically limiting ground current, RCD is likely to not provide the kind of protection you need because a ground fault may not be seen soon enough, so you must use a ZSCT. But for solidly grounded systems, RCD works fine, and RCD requires CTs on each phase.
The reason you might see both a 2 CT measuring system and a 3 CT GFP system in the same equipment is because you don't want your RCD CTs being tricked by metering requirements that might cause an imbalance on the secondary side. When you have a metering switch to select your meter to read phase A, B or C, the secondaries of the CTs are briefly shorted with overlapping contacts to avoid an open circuit. That action of moving the switch might cause a nuisance GF trip if an RCD GFP relay was connected to them.
