RAM modules should always be used in pairs. Two identical modules is ideal. If you use three modules, then the third module is only running in single-channel mode, dragging down the performance. And the memory system doesn't like this imbalance, as your other two modules will be different, but there can only be a single set of parameters for the whole memory system. But then it would have to be some compromise that tries to make two RAM modules with completely different ICs (memory chips) happy at the same time, even though they need different parameters from the board.
And i'm not even primarily talking about the speed and timings here, i'm talking about the electrical parameters. So the IMC (integrated memory controller of the CPU) has to do "the splits". It sees two different kinds of modules being used, with quite different requirements for certain parameters on the memory bus, but it can only set a "middle ground". But there is no good middle ground between such different modules.
Mixing different RAM is like this:
When you really want it to be completely even, like this:
So it's always best to use a kit of two identical modules only. See here for more explanations:
RAM explained: Why two modules are better than four / single- vs. dual-rank / stability testing
In your situation, definitely remove the single 8 GB module, use only the new TeamGroup kit of 2x 8GB, in slots A2 and B2 (2nd and 4th from the left).
