Hi
If the lease says a three way equal split then you are bound to that. Otherwise the premium should be apportioned by the size of each unit within the property assuming the build specification is the same throughout.
The insurer will rate the whole building on the basis of the highest risk which presumably in this case is the shop.
If your unit is separated from the shop by walls and floors/ceiliings which are fire retardent up to a certain level/number of hours and there is no communicating staircase or other void that fire could travel along then you might have an argument for having your property rated differently (I am not sure what the number of hours is the insurer could tell you) .
Finally, the policy will almost certainly include Property Owners Liability - again, unless specified in your legal agreement with the owner I dont see why you should pay a proportion of that.