Do your your own ratings on every horse in the race. If you have a standout rater use it as a banker if you have a couple put them both on top and your other selections in the 2nd and 3rd leg. eg 2/7/7
When rating them make sure you take into consideration the jockey and Barrier. A good jockey can jump from anywhere and can get some longshots into placings and a good barrier can sometimes get a lesser class horse into a place if it has a good run. These give the trifectas great value.

I've been to busy lately so i've just been banking my selection with the field. It sometimes backfires when the favourites come in 1st,2nd and 3rd but i have a win bet that covers any loses i might get on the tri.
I usually take trifectas when there is value .eg atleast 8 horses in the field and my ratings dont have 3 or 4 horses which are all in the betting that are way ahead in ratings than the other horses in the race.

Good luck ..