Brother MP,
The Trifecta pays for the first 3 horses come home. if you box 3 horses, it will cost you $6.00 - the more you boxed, the more it costs.
To box 8 horses like I did would cost $336.00 (it works like 8 x 7 x 6 = $336), I usually bet half a unit = $168.00 and get 50% of the winnings.
You can certainly use your $300 bonus bet on the box trifecta and it will give you 89.28% of the winning. and then they will deduct the $300 bonus itself from the winnings.
In yesterday's case and Sportsbet will be using the Victoria toll, the trifecta was paying $2,953.40 from which your 89.28% will pay you $2,636.78 - $300 = $2,336.79 !
Not bad from using the bonus bet buddy !!