Improve uniqueness of block for nonce creation
Blocks include more than just those 3 variables. One of the most important ones is the merkle root. This is the hash of all of the transactions in the block, in a particular order. If different transactions are included or they are included in a different order, that merkle root is going to be different. So if each miner has a different merkle root, then they are going to have their own set of nonces to work with. That is what actually provides uniqueness here.

Miners all have different merkle roots. This is because the merkle root includes the transaction that pays the mining reward - the coinbase transaction. Since every miner wants that reward, they will have a coinbase transaction which is different from every miner as it will include their payout address, not someone else's. So by virtue of having different payout addresses in their coinbase transactions, the blocks that miners are working on will be unique and different from other miners.