Initially we had the X resistance, but then I opened the circuit in the terminals of resistance X, and I moved the resistance X to the equivalent circuit (my teacher says this is a shortcut of how doing it). My doubt is about calculating the equivalent resistance: it is obvious that R1 is in series with R3, and then they are in parallel with R2. But, since I opened the circuit, the current that are going to those terminals is going to be zero, so there are no nodes, because the current doesn't divide. So, why couldn't I say that R1 and R2 and R3 are in series, or R1 and R2 are in series and these are in parallel with R3.
What's missing me? Doesn't the current in terminals is zero?