I like to get in pretty close before I fire. Usually 10000 yards or less for conventionally propelled torps. Anything that gives my opponent less time to react. If I'm in a Russian boat, I'll definitely be trying to use Shkvals, and you have to be in real close for those!
A while ago, I did a quick derivation of maximum engage range for a torpedo and minimum safe distance from being shot at by an opponent. I'll link that video here:
It's not a precise calculation because engagements are not precise things, so there's some variable conservatism built in to the resultant values.
On spread technique, that actually sounds like a decent idea for long-range Shkval shots.. I would probably try and do it with two torpedoes, though. Three is a lot! Especially if you're in a boat that only has four tubes. I can't remember the last time I shot two torpedoes down a bearing. Even if one torpedo doesn't hit, at least I've hopefully shaken up the other guy and got him to run away somewhat, allowing a reset to the engagement that will hopefully result in me having the upper hand.