Assume that there are only three mining pools, all with the same hashing power, mining for bitcoins in the bitcoin network. Which mining pool is most likely to get the block reward? Do you all think that it will be equally likely? Since they have the same hashing power and working on the same candidate block.
Yes, the same hash power means the same probability of mining a block.
But there can be other factors like connectivity: if a mining pool is facing internet connection problems between it and its members, it may fail to use that hash power effectively. P2Pool FAQ talks to its members about the importance of a good connection: http://p2pool.in/
Also, if there is an internet outage soon after it mines a block and it fails to broadcast it, the block risks been orphanated.
But it's not possible to 3 miners to work on a same candidate block, each block they are trying to mine will be necessarily different. I think you mean 'on top of the same block'.