re-entry after three days possible for conference?
Yes (even within minutes).
how big are the chances that I would be refused entry anyway?
Negligible, provided you comply with other requirements.
The USA does not impose a fixed duration hiatus between ESTA visits. Basically they have indicated "we trust you well enough to behave yourself" so tend not to turn ESTA visitors away regardless of time intervals.
I'm not sure exactly the basis of the statistics but CBP claimed that on a typical day:
1,048,632 passengers and pedestrians
308,234 incoming international air passengers and crew
54,236 passengers and crew on arriving ship/boat
686,162 incoming land travelers
72,179 truck, rail, and sea containers
282,252 incoming privately owned vehicles
• Conducted 924 apprehensions between U.S. ports of entry
• Arrested 23 wanted criminals at U.S. ports of entry
• Refused 367 inadmissible persons at U.S. ports of entry
• Discovered 470 pests at U.S. ports of entry and 4,548 materials for quarantine — plant, meat, animal byproduct, and soil
9,435 pounds of drugs
$356,396 in undeclared or illicit currency
$3.7 million dollars’ worth of products with Intellectual Property Rights violations
• Identified 600 individuals with suspected national security concerns
• Intercepted 26 fraudulent documents
and so on.
That is, refused 367 "inadmissible persons at U.S. ports of entry" out of what seem to have been over 2M who sought entry. Perhaps out of 308,234 "incoming international air passengers and crew" would be more meaningful. Even with crew and US citizens deducted, it still seems the chances overall may be around 0.1 or 0.2% before deducting from the 367 those that were dishonest, idiotic etc (quite possibly to leave just a handful with some justification for feeling umbrage at refusal).
Those 'above board' with a visa have excellent chances of being allowed to enter. Those for whom the chances are less good are ones who are being dishonest, abusing the system or are just plain stupid. Obviously CBP have no machine that separate the two groups so, where not already tipped off, sometimes will need to ask a few questions to gain clarification. What they are looking for is untruths – should you be questioned, be open and honest and you have nothing to fear.
In any case one short interval is nowhere near the warning sign given by many medium intervals. In effect, we all are given a 'second chance'.