Why did it take 3 days for dragon since detach from ISS till splashdown - while Soyuz would take 3 hours ?
Axiom's Ax-3 had a notably long post-undocking orbit time and no clear explanation has been given for why that would be. It was actually just under two days, not three -- undocking was February 7th at 9:20am EST, and splashdown was February 9th at about 8:30 EST, which is just short of 47 hours. It's possible the paying passengers simply wanted to have a little more time in the capsule and Axiom was willing, but if I had to make a bet, I would put my money on weather concerns. Ax-3's departure from the ISS was already delayed several days due to storms in the landing zone, so possibly there were ocean conditions they considered less than ideal and waiting an extra day gave it time to clear up entirely. Ax-1 and Ax-2 both took close to 12 hours to return, which is in line with other Crew Dragon missions.
Ax-3's unusual behavior aside, you are correct that Soyuz has a notably shorter undocked time on orbit than Dragon does. Soyuz makes the trip up and back down in about three hours each way, while Dragon routinely takes 12 hours or longer for both the pre-docking and post-undocking portion of the flight. Why is that?
We don't know all the details, but there are a few things we know, and a few things we can guess.
As a general thing, the fast intercept and return requires more trust in the capsule, and NASA is notably risk-averse. If you plan a three hour trip, you have to be pretty sure there won't be any problems. A more leisurely mission plan means a lot more time to make corrections, fix any issues that show up, and a generally slower pace to operations. Soyuz is an older technology with a huge number of flight hours, while Crew Dragon is still relatively new. So that's point number 1: Dragon may not yet be considered ready for the high-stress fast intercept and fast return tracks.
But second, Dragon is a more comfortable ride. Crew Dragon has a huge interior space compared to Soyuz. It's the difference between putting four people and all their stuff in an SUV, and cramming three large people with all their luggage into a Volkswagen Beetle. The crew in a Soyuz can barely move in the descent capsule, so staying in orbit for many hours or even days is just not feasible. I'm sure 48 hours in a Dragon is still pushing the bounds of comfort, but it's much more doable. (Technically, there can be more space provided by the Soyuz orbital module, but that's usually full of cargo on the way up, so there's not really anywhere to go that's less cramped than the descent module. I'm not sure if it's less cramped during descent or if they pack the orbital module with garbage so it can burn up on reentry.)
Third, a longer orbit time makes scheduling easier. The quick path requires a fairly narrow phase angle between the launch or landing site and the ISS, so your timing is critical (I believe it's a five-minute launch window that comes around every few days). That is to say, to get down from orbit in three hours, you have to release from the station at exactly the right time, with little margin for error. If you plan a twelve hour travel time, then there's more flexibility to launch or undock whenever it's most convenient for the people involved.
So putting that all together, a slower path might show less trust in the technology, but if it isn't a major imposition on the humans involved, it also makes for a more relaxed trip with a lighter workload on the crew, as they have more hours to spread out the tasks the need to do, and more flexible time windows.