DWV stacks, S Traps, Oh my

My home was built in 1913. I have a bathroom on our first floor that has a sink, tub, and toilet. In the basement (just under the 1st floor) they converge to the soil stack which is vented to the roof. No fixture is more that 3 feet from the stack.

From what i have been reading trying to determine the cause of my slow sink and tub (with a drum trap), this is not vented well. Is this technically an S Trap since the drains go all the way to the stack before it can vent?

i am trying to come up with the best plan to improve my drainage. My sink is always a little slow, but for some reason worse in the winter. It glubs when the water has finished draining. Even the trap under the sink i think is an s trap. The tub barely drains at all, but hasn't since we moved in and i probably chalk that up to the drum trap. I may end up cutting it out if i can't get the cleanout cap off. The toilet seems OK.

I am most concerned about my sink since twice in the last two weeks a family member has overflowed it - so the overflow drain isn't working well either. Any idea of what i can do to help my sink?

An S trap, or lack of vent, does NOT cause slow drainage. Either can contribute to gurgling noises, but not slow drainage. You probably have a clog.

If the sink does not drain, the overflow will do NOTHING to keep it from overflowing, because it will be plugged up also. Just because you do NOT SEE a vent, does not mean you do not have one. We would have to see your system to figure out what the problem REALLY is. NOT having a vent, only causes a drainage problem when there is also some other problem.