can i decide which block to mine ?
You cannot decide which block to mine, but if you mine a block, you can decide which transactions to include.
Realistically, you would be extremely unlikely to mine a block unless you have a
huge amount of equipment and massive farms of ASICs. You would have to join a mining pool, and they would decide to mine the transactions with the highest fees, so you would not be able to choose which transactions to include.
lets say i did a transaction and right now its in the unconfirmed pool. (for 3 days now)
can i with my miner(s) pick it up and confirmed it?
Theoretically yes - practically, it's very unlikely.