A tri bet is a special type of margin bet that splits the range of possible points margins in a match into three outcomes. Team A to win by 15.5+ points, Either Team A or Team B to win by under 15.5 points and Team B to win by 15.5+ points. The size of the points margin range depends on the sport.
A roundabout bet is made up of 3 singles and 3 any to come (ATC) doubles to twice the stake. If the single bet wins, twice the stake is then used on a double for the other two selections. However if one of the horses odds are less than evens, the full return will be used on the additional bet.