The kidney is not such a large organ to "steal" blood from other organs, there are no negative aspects, unless it presses on a neighboring organ. But the body has an increased ability of kidney function.
There are more potential disadvantages. The third kidney must somehow be supplied with blood, which means there will be extra vessels. The presence of extra vessels can be accompanied by robbing of other organs, or two other kidneys, or they can be squeezed themselves or squeezed other vessels. Constant ischemia can provoke infections and become a factor in the development of oncological processes. And the third organ, wherever it is, is rarely a full-fledged version - they are either less or underdeveloped, which means they do not fully function. Pros .... I don't even know what they might be
If the "extra" kidney functions on a par with the others (if at all), then the amount of filtered fluid is evenly distributed among the three. If one of them fails, then the other two will be able to cope with the filtration function more efficiently.
But, in fact, there are no clearly proven advantages or disadvantages of having a third kidney.
One man had three testicles and he loved to brag to all men: "We bet you and I have five testicles for two?" Once he asked his favorite question to a nondescript little man, and he answered: "Do you have one thing, or what?" ... Or brag like the guy from the anecdote above. It is only difficult to check without an ultrasound.