if the card is at 100% load, i don't think you will be able to get it even hotter.
The only thing that will happen is loss of performance, with 3 screen the memory usage will be much higher and the performance drop will be much higher.
i don't think the gpu will be hotter than that, the idle temps will not change much because the gpu usage in idle is nothing significant, so if the gpu usage is 1% in idle, will be something like 2% with 3 monitors... is very low.
The TDP for the card is about 200w. As the other poster mentioned unless you're overclocking the card it's going to pull around 200w at max load +/- a couple watts. Running 2 or more displays will usually prevent the card from going to the lowest 2d clock speed. So at idle it may run 5c warmer or so, basically it’s a minor difference though so I wouldn’t worry about it.