As a newbie bullet caster myself, I am FIRMLY in the slug the bore first camp. I've been fighting leading for several months. Finally, I slugged the barrel and found each of my 9mm's have different diameter barrels. So, bullets that don't lead in one gun will lead in the others.
Find a bullet that is .001 to .002 larger than your bore. Hardness is WAY overrated. Quite a few folks cast air cooled wheel weights (which are 5 to 7 BHN less than the formula says is required) without getting leading. I can't remember what the formula's BHN was for 9mm but it was probably double what I'm shooting.
Finally, lube seems somewhat important too. I've not yet had luck with liquid alox lube, while the same rounds work fine (i.e. no leading) when using NRA 50/50.
Buy an Outers Foul Out unit to delead your barrels while you sort everything out.
In answer to your specific questions:
>> Can I obtain high enough velocity for this to be a problem with at 3" gun?
Yes. If your bullet is undersized, you can get leading in a .45 ACP (much lower velocity than a .357). BTW, its not really the velocity so much as the pressure. A 9mm has higher pressure (i.e. harder to get no leading) than a .357.
>> At what velocity does this become a problem typically?
At almost any reasonable velocity if the bullet is undersized.
>> How quickly does leading become an issue...5, 50, 500 rounds?
Depends on how out of whack your bullets are. I've had leading in as few as 20 rounds and it was SIGNIFICANT leading
>> What is the symptom of leading(decreased velocity, increased pressure)?
Never checked on these type symptoms - only saw a thick coating of lead in the barrel when getting home. But, you most likely will see a lack of accuracy as the lands lessen due to lead build up. Also, since the bullet is undersized to begin with, you will see poor accuracy and possibly keyholing.
>> If I clean my gun after every range trip is it even anything that I need to look out for or care about?
Depends on the amount of leading. After some of my outings, I sure wouldn't want to put additional rounds down the barrel.