pouring 3 litres of water inside it with the lid slightly open and waiting for the water to boil.
Good. You definitely want to get everything up to local boiling. The amount of water is reasonable for many canners, but depends on the model and size.
After it boils I wait for around 10 minutes so that any air trapped inside the canner is squeezed out.
If you haven't closed the lid, this
serves no purpose doesn't do anything about "squeezing out the air". However in many canning recipes there is a boil period with your jars in the water before the pressure phase. If you had room temperature food in the jar it can take a while to come up to temp. A 10 minute boil gets the food heated up so the (later) pressure cook phase doesn't have to last as long.
Afterwards I close the lid. Can I assume that if I reach 1 bar pressure inside the pressure canner I will have 120 degrees Celsius inside it?
The operating pressure indicated on a pressure cooker is the target additional pressure over your local atmospheric pressure that should be maintained during cooking. (The operating pressure may be lower than the maximum on the relief valve).
At 700m altitude, the ambient standard pressure is 930mbar (but can be lower with low pressure systems). Let's say on a rainy day that it's as low as 900mbar for you.
IF your cooker operates at a true 1 bar, then the internal pressure will be 900 + 1000 or 1900mbar (27.5 psi / 1.87 atm) absolute. Using a calculator you can find that water will be at 118.6C/245.5F when it is boiling under such pressure.