Your first circuit shown does not work. Using just an RC circuit means that the SCR must be a 'sensitive gate SCR' with very low trigger current. The RC charge is complicated in that the capacitor will charge to the gate break over voltage on the negative half cycle (typically 6-8 V) and only charge to the forward gate (typically 0.7 V). This makes it very difficult to calculate the RC required for any given positive turn on point, and when the reverse gate voltage is exceeded it depends on the SCR construction what will happen (it may clamp or avalanche).

Your second circuit shown does not work either. While using the DIAC improves things, in this version the negative half cycle is almost guaranteed to avalanche the SCR gate. In the positive half cycle, the RC network can be calculated for the DIAC trigger point (typical ranges are from 25 - 40 V). Once the DIAC triggers then the capacitor is discharged into the SCR gate. Pulse currents may be in the 500 mA plus region. You also would typically place a resistor ( 1- 2k Ohm) across the SCR gate.

Have a look at some typical SCR's on Digikey, you will see for the range I chose the gate current ranges from 200 uA (sensitive gate) to 60 mA.

Your third circuit will work, but it should have a resistor of 1-2k Ohm across the 1N4148 Diode. Here the SCR is triggered by dumping the capacitor charge into the gate for the positive half cycle. For the negative half cycle the capacitor charge is clamped by the 1N4148. This means for both half cycles the capacitor is charged from the discharge level (typically about 1.5 V) achieved at the last trigger. Now you can work out the RC network reasonably accurately.

It's worth noting that you can use a single SCR to achieve dimming on both half cycles of the mains voltage by using a bridge rectifier.

simulate this circuit – Schematic created using CircuitLab

This type of circuit works well in the 240 V AC world where it provides a good range of adjustment. It's not quite as good in the 120 V AC world since the adjustment range is not as good. For example if you used a 40 V DIAC and had a bridge rectifier with 1.8 V drop, and another 0.7 for the SCR gate the total is a 42.5 V drop from the AC input signal. That means that you can never set early turn on less than 42.5 V which is about 25% of the peak value for 120 V AC but only 12.5% of a 240 V AC waveform.