Great choice in both caliber and the gun...you can't go wrong. As to the holster, a good fitting one will not need a strap, flap or thong to retain the gun, so long as you stay off bucking horses, (don't ask how I know).
I like the old Tom Threepersons variety but without the closed muzzle end...I prefer it open to shed water and woods duff etc. The ones produced by El Paso Saddlery can't be beat for beauty and function, and do come with a strap if you so desire. Your mention of a "flap" is not all that bad an idea, preventing water incursion among other things, but will obviously slow up your draw.
It's a fine choice you've made (I carry a Smith M60 with a 3" bbl. for the most part when traveling to the "Big City" and also for many chores here on the farm.) BTW, a good combination if you reload is Skeeter Skelton's old every day carry load: a good LSWC pushed along by 13.5 gr of 2400 in Magnum brass. It'll give you over 1050 fps from your 3" SP101 and I suspect, superb accuracy. I've yet to find a .357 that doesn't like that load combination.
As always, that load should be worked up with due regard to pressures, but works to perfection in my .357's.
Here's a pic of my home-made version of that old staple for DA revolvers, albeit with my wife's M36 saddled up.
Best Regards, Rod