Hmmm. Well 3G is a lot of force. That said we can call an instant of 14G completely safe and John Stapp would shortly chime in with a couple figures. 32G being safe, 40G being tested safe to the extent of no lasting damage, and 46G safe by extrapolation. With no known upper bound on survivability. That said 32G is probably enough for us. So if we're swinging for 3 seconds at 3G and make a stop in 1 second, we're experiencing 9G. Which simple math makes that 32G÷3G=10⅔
So you could swing for 10⅔ sec per swing any way you wanted and if you stopped in a second you'd be fine. No extra contraptions required for "health reasons".
But wait! G-forces change depending on direction of your swing! So a perfectly ground-parallel swing might experience the full Gs all-the-way-through but a regular swing would have some varying Gs. So you could potentially swing even longer than that! If the stop was at the base of the swing where there was peak force, then that means you could have up to double the swing time if you were stopping at the base! And as much as you wanted if you were stopping at the top!
Then again a "Spider-man-style" landing might have lower G tolerances than purely feet first given there is a forward component. So maybe a little bit less than 10⅔...
But maybe you're more curious not in the health implications but in actually achieving this stop in one second. This paired with this would seem to suggest a human with gecko grip could stop with just under 20Gs of force on a dime for some landings at least. Add in some G-force activated locking joints in an light-weight exoskeleton and you could pretty simply come to a screeching halt at 19G it would seem. Pushing the bar to 32G or higher would maybe require better tech than we have now to remain unnoticeable, or maybe the same concept but engineered better than a gecko could get us there. Granted 19G is still a minimum of 6 sec of swinging which is a lot longer than what you typically see Spider-man doing.
Ah, btw technique is pretty important. If you're stopping faster than 1 second because you've collided with the ground on the down-arc of your swing... well your stopping time is for sure less than 1 sec and your Gs are going to sky-rocket and turn into pressure and kill you. So a crash at 3G is equivalent after 1 sec to a 65mph crash. So you'd probably only be able to come out of a ground crash at 3G if it happened within a fraction of a second (spit-balled from some median death heights, etc. go look here if you want a good starting point). Since you're worried more about a collision in that case than G forces the question is different. But yeah, no downward arc stopping, especially if your arc looks like free-fall.