If I understand correctly, you cannot just start a tx, but you'd have win the block ?
Yes. That is correct.
1). What if I only fill my transaction on a single block and confirm it with my own 2,3, 4, 5 of my own miner.
That is possible also, but the probability of you winning a block is very low.
Unless you have a large hashrate.
You can fill only your transaction in the block, you can choose not to include any transaction except the Coinbase transaction if you wish (like Antpool does)
Doesn't it only take no more than three confirmations for a tx to be validated ? I know it's an extremely simplistic question, but I just want to be and advocate this practice
If, by a stroke of luck, you actually solve the block with your transaction included the transaction will have 1 confirmation (yours).
Any other block any miner mines will build ON TOP of YOUR confirmed block and the number of confirmations will increase with every valid block mined on top of it.