Sleep training, controlled crying - any success??

Ki

We're on day 3 of sleep training our 3 month old; very gently - patting / gradual retreat method. Despite this she cries a lot and really wants to suck, but that's the habit we're trying to break. I know it might be a bit young but I'm not coping with all the sleep loss... Does anyone have any success stories with sleep training or controlled crying? We only ever leave her for a minute or two but in that time my heart breaks!! First time mom.