Ha haha

I have 3 kids, 6,3 and 7 months.

Ask a child if they would do something you want is asking for trouble, don't give your child a question in which they have a choice.  If you wish for them to tidy up, put their coats on, shoes on, to stop misbehaving, TELL them.  

This isn't a question that they can answer, you are instructing them to do what you want them to do, failure to do so will see the consequences of their actions.  I remove the toys after I tidy up, lock them in the garage for a day (next day), the next time I tell them to tidy up, they know by refusing they will face a day without toys, and tidy up without any quibble.  It takes several goes before they realise what's happening.

If they refuse to put their coats on, either myself or my husband will get in the car with the child that put their coat on and go out, the offending child remains at home.  

I know that sounds harsh to do, but in the long run it makes it easier for myself and the children if they do as they are told.

My children are very resolute in expressing their opinions, I think they know the difference between voicing their opinion on something and refute what you are telling them to do.  

I just think your child is now testing boundaries and see how far they can go, it is not about you stamping down on them voicing their opinions, it is them seeing just how far they can push you.  My eldest is 6 and phased out of this behaviour as he knows where the boundaries are, hence why I think that is what your child is doing.  Happy Parenting!

JJ