Dear, it is not necessary to have the same brand but what is required is that both modules should have same timings i.e CAS Latency and bus speed and that's it. However, it is recommended to use same brand's modules to ensure that they are compatible and that this configuration works.
Yes it can work with 20GB only if they have the same timings. If bus speed is not the same and both modules (in your case three modules) are picked by the system then it will restrict all the RAMs to the lowest bus speed.
Here is my recommendation: You may check it as it will not harm your computer.
Happy Hunting!