It is about time to change the tires after 75,000km in most cases, though they could be good for another season.
3. 买三个新轮胎,加上原有的备胎把四个轮子都换了,用一个旧轮胎做备胎.
From an economic point of view, this would be a good option. But the three new tires and the existing one must be exactly the same (same brand, same type, etc.). Keep in mind that the age of tires matters; Even a tiire is never used, it could be useless after ten years.
4. 买四个新轮胎.
From a safty point of view or to save yourself from any unexpected issues, this would be the best option.
Unless you know how to check if tires are good without any potential damage, I'd suggest not to put on used tires.
Tracy, If I were you, I would buy four new tires for the SUV and have the wheels properly aligned. That will save my life and my trouble. Think about, one tire of the four was damaged. It's the indication that the tires need to be replaced.
The manufacturer says the tire can run 100k or 120k km, but it defines the perfect conditions. In the real life, you never get those conditions, such as road condition (no pot holes, no sharp edges, no sharp steel pieces), vehicle condition (good alignment, steering and suspension systems work properly), the driver's driving habit (no speeding, no fast take off, no sharp brake). If any of those conditions was not met, your tire life is reduced.
Let say, each tire costs you $150 and it can run 100k km. If you scrap the tire now, you used three quarters of the tire's life. That is good enough. For four tires, you lose one tire's value for $150, however, that $150 can save you a lot of trouble and it may save your life (some accidents were caused by defect or damaged tires).