The easiest way to think of it is that each 3 horse combination is one 'selection' in a leg. Hence if you took a banker (horse A) with 4 other (B,C,D,E) you have 6 'selections' namely ABC, ABD, ABE, ACD, ACE and ADE (remembering that these are trio selections not trifecta selections so the three horses can place in any order).
Hence for the bet you suggested you would be taking 6 selections in each leg and the number of combinations would be the same as taking a treble with 6 selections in each leg i.e. 6 x 6 x 6 = 216 so to take the bet for 5% would cost $10.80. The most this will pay you is 5% of the pool if you get all three legs (i.e. $12.5k this weekend assuming the pool is $250k) even if you are the only winner (a very nice collect but hardly the life changing result the endless TT promotion implies) and if there is more than a full winning unit the declared dividend will be less than $250k and again you will only get 5% of the declared dividend.
To get the concession you need to select the trio in the first two legs. In your example if you selected the first two legs, but not the third leg you would get 30% of the concession dividend, as while you only have 5% per selection you have 6 live selections going into the last leg so you have 6 x 5% units. The one exception to the percentage payout for the concession is that provided there is at least a single live ticket after two legs, the full concession pool is paid out, so if you happened to have the only ticket live after two legs and this was only for 10%, you would get the full concession pool and not just 10% of it (I think this happened one weekend).
I think what I've said here is correct, but happy to be corrected if it's not!