A constant pressure can be converted to a constant flow by a restriction, or a long thin pipe of suitable dimensions. A higher pressure with more restriction gives a more constant flow.
At one end of the scale you could just regulate the flow from each tank manually so that when the third tank reaches a certain level (full) it is the correct mixture, and the flow is stopped by two on/off valves such as solenoid valves or whatever. This is probably the simplest and needs no fancy controller. These flow regulators might be ball valves, though there are a variety of types that might suit. The names of valve types might change in different places. Ball valves are often used with kitchen or bathroom cabinets behind the scenes. You need to select a valve for suitable materials in contact with the liquids, the flow and the pressure and the fittings. The first link is one example. The supply tanks would be well above the receiving tank to provide sufficient pressure, with the valves near the receiving tank level.
A solenoid valve provides an on and off control. Examples are in lawn irrigation systems and washing machines. They would very likely be operated by a relay because they are designed for an AC voltage. The relay is operated by some sort of controller. There are many types, chosen for the materials in contact with the liquid, normally on or normally off, the voltage/supply, the flow and the pressure and the fittings to consider. The second link shows an example.
There needs to be a water level sensor to detect the tank is full and shut off the flow. It may even be possible to operate by a time if the flow is constant.
Be aware that solenoid valves are often operated by the pressure of the supply, so you need a direct acting type probably (second and third links).
It may be possible to adjust the mixture automatically if you have an appropriate sensor in the receiving tank to tell you the concentration. This might be done by regulating the flow from one or both of the supply tanks. Adjust them so the mixture is approximately right from the supplies, and then the supply/s can be regulated, perhaps on and off with the solenoid valves till the mixture is right according to the sensor.
Another approach may be to first fill with one liquid to a certain predetermined level, then add the second liquid to a second predetermined level. This needs two level sensors to control on/off valves, or perhaps two timers will work if the flow is accurate. The choice of level sensors is large. If the volume is small it might make sense to weight it (look up load cell).
There are flow control valves that can be adjusted proportionally to allow a steady mixture, manually or automatically using a motor and controller. A signal indicates to the valve how far open it should be, and the system controls the mixture through this. There are also gear pumps that can be used for metering the flow. Another approach is to measure the mass or volume flow using sensors and control the mixture according to that. These are more expensive ideas but may better suit your process, depending on the desired accuracy. The more expensive methods tend towards quicker filling or more accurate mixtures if these are important.
The control system might involve programming your own micro-controller to get a desired degree of automation and control using a sensor or sensors.
For proper automation there may need to be controlled levels on the supply tanks, and a check
that the final mixture is correct, and the container is full. Alarms for problems. Manual shutoff valves? I think best to start with a manual system using ball valves as low cost flow regulators, and go from there. Otherwise the system with two level switches in the receiver tank, one for each supply.
Ask sales people for technical help. The suppliers of the more expensive valves have technical sales reps.