Septic tanks are always "full" to the level of the outlet, and the inlet pipe is slightly higher. Then when new stuff enters the tank, it displaces a volume equal to itself and that displaced volume overflows out through the outlet.
Are your toilets located one above the other? If so, there might be some kind of obstruction where their discharges come together. Or, maybe your overall septic system is "slow" for some reason and cannot handle the rush of a flush as well as it can handle the slow flow of a sink or shower drain.
When I check my own system, I always begin at the distribution box to be sure the septic tank has a place to drain, then I check inside the outlet end of my tank. If both of those are okay, the problem has to be somewhere between the toilet (or other fixture) and the septic tank.
Most important of all is to be sure you have a filter at your septic tank's outlet. A tank can be easily cleaned and pumped, but a plugged-up drain field can only be replaced. A filter at the outlet of your tank protects the most crucial part of your system, the drain field.