So what was the bet??? It's difficult to advise you with such little information.
How much was your deposit?
What were your bets? - you said you made the same bet 3 times, this may be the problem especially if you were wagering with the bonus. Some bookies do have a term that states placing multiple bets on the same event will be void. This is usually in the terms of bookies that have a maximum wager limit on bets made with a bonus (there isn't that many around, but I have seen this term before).
For example -if the term states you cannot bet more than 50% of the bonus on one bet / outcome, then if you have a bonus of £20 your max wager must be £10. If however, you were to wager on the same outcome inplay or place additional bets on top afterwards, these will still be recognised as one bet greater than the terms allow thus being voided by the bookmaker.
If you made a mistake, then you should have contacted customer services and let them know, they would probably have been sympathetic. Not doing this would make it look like you intentionally circumvented the limits in the terms.
While I'm sure you will say they should have software in place to recognise this, it is either not cost effective or simply a lot more difficult to implement than it sounds.