Doubles (3 Comb) is a type of accumulator. It means if your have like 3 events on 1 bet slip, instead of the normal accumulator which multiplies the odds of the 3 games to get your odds product and then multiply the odds product by your stake to get your winnings, doubles (3 comb) because of the "term" double the system would pick any 2 of the 3 events in 3 different selections.
The system then arranges your 2 combinations picking any 2 of the 3 matches in the pattern below
Combination 1 ..... Match A and Match B
Combination 2 ..... Match A and Match C
Combination 3 ..... Match B and Match C
Odds product for A and B X stake (100) = winnings for the combination
Odds product for A and C X stake (100) = winnings for the combination
Odds product for B and C X stake (100) = winnings for the combination
In doubles (3 comb), Matches A,B and C would be on your slip but the stakes will be calculated individually for each combination. So if you enter N100 in the stake box, your total stake will be N300 meaning you will be charged N300 if you proceed with that bet, this is because you are placing a doubles (3 comb) bet and your total stake will be calculated at N100 for each of the 3 combinations. Unlike the normal accumulator, you don't have to win all the bets on the slip to get returns, you need to win just 2 predictions for you to get at least a return for a combination.
If you win match A and B then lose match C, your stake of N100 for the combination A*B would multiplied by the odds product for match A*B and you will be credited for that. You will not be credited for combinations A*C and B*C because you lost your prediction on match C.
An example of a doubles (3 Comb) bet is displayed in the attached file.