Is one seal stop leak 3x sealer safe to use in a a 2010 santa fe 3.5L engine?

No, do not put in additives that could clog up oil passages and journals, instead, get some engine cleaning products, follow the instructions and carefully clean your engine and engine bay. Then in a few days, you can park the SUV over some cardboard to see if you can localize a leak or you can see where oil might be leaking from above with the engine not covered in crud. It is likely the valve cover gaskets, a cheap fix, but you may need to check the sparkplug boot areas for seepage damage. Often the oil is just from spilling some over the years and it builds up with dust and more grime. You could get some blowby from poor EGR and PCV valves, but those conditions will normally throw a code. 

Good advice is never to introduce products designed to "cure" issues by plugging stuff up, it's better to fix the underlying issue(s). If you have a known hole in your radiator you get the hole brazed if possible or get a new unit. Don't put  "seal" conditioners into transmissions because they soften everything and you can easily ruin a transmission without knowing the real cause. Those are just things people seem to do all the time and regret. 

Fix the car properly.  Find the leak and replace the bad part that is causing the leak 

i would ask a mechanic about it