In olden times, it was common to only provide two phase overcurrent relays along with a ground relay. As long as all the relays work as intended, this provides adequate protection against all types of faults. Obviously three-phase and line-line faults are covered. And a line-to-ground fault in the phase without a phase relay will be detected by the ground relay.
For new work, you should go with overcurrent protection on all three phases. Since most new relays are three-phase digital relays anyway, that isn't an issue. For retrofit work, the problem will be the missing CT. You should try to add a third CT is possible and go with full three-phase protection.