I have my brother's Honda ATC 185S 3-wheeler at my house. I was going to remove the tire and grease the chain up but I don't know how to remove the tire. Do I remove the 4 nuts on the inside and 4 nuts on the outside of the rim or take the pin out of the big center nut and remove just that one?

 

Those rims are probably 2 piece. Look at the back of the hub see where the wheel studs are and remove the corresponding nut on the front once you figure out one then it's every other nut.

 

You could also simply remove the rubber plug on the chain guard and lube the chain
without removing the wheel
Just my 2 cents
Ray