Toddler saying no

Our 3yo will sometimes just simply say NO during instruction. I may be getting this wrong but I have been calmly taking him to the calm down spot with brief explanation of what to expect, and from there everything will escalate far beyond where we started. He will throw a tantrum and require a soft hold until he eventually gets calm and is ready to cooperate. His time in the calm down spot IS becoming less and less, BUT in those moments where he is simply saying NO calmly, it seems like the calm down spot sometimes leads to him LOSING control. Am I totally missing a step or a tool?