Two-Wire Control of Three Phases. You can connect two SCRs in just two of the three secondary lines and use fast-cycle control. You buy only two SCRs and rate them for the line current and line voltage. This arrangement rules out both phase-angle control and load balancing because of you cannot use a neutral conductor.
SCRs Controlling in the Primary. The general rule here is to use phase-angle control to avoid the high inrush problems of fast cycling noted in earlier parts of this series. Two preferred connection would be: