With small capital you still want to follow the same structure philosophy. Find yourself a single and then narrow the 2nd place horses and make sure to go deep in 3rd. Any horse, no matter how bad they look on paper, can get up for 3rd. Also look for tendencies of horses. If you have a horse that has 21 career starts with 1 win, but has 4 places and 6 shows then that is a good horse to key in 2nd and 3rd. So basically you flip your key to 2nd or 3rd. I would not invest tons in a trifecta. I would not bet anymore than $20. That is a single with 5 with 5. That is a pretty good bet. When I used to bet trifectas, I would structure my tickets like this.

x
with
xxx
with
all

The cost depends on the field. But once you get a bank roll, you will be better serve by betting to win.