Every block header has 3 patricia trees.
It contains a hash of the root nodes of 3 tries, not the entire tries. The tries themselves aren't stored in the block, they're stored separately in the state data. When a new block is received by a given node in the network, it applies the state transitions described in the block's transactions, and updates its locally stored state data accordingly. The hash of the root of its local tries should match the field in the block header.
But, what addresses are in the paths of the State Patricia Trie of a given block? All Ethereum addresses or only those mentioned in the transactions of that particular blocks?
The hash of the root node of the "world" state - i.e. the combined state associated with all addresses, as organised into a single trie - will change if there is a change to the state associated with any single address.
So regardless of the number of transactions included in a block, the hash of the root of the entire state trie will change.