The step sequencer can't handle inserting triplets into an otherwise "straight" pattern. It's modelled on hardware sequencers, restrictions and all, and that's one of the things most hardware sequencers can't do either. Step sequencers aren't really intended to produce realistic imitations of humans....
One work-around might be to create sequences of just the triplets and insert them into the track as necessary between the other patterns. I tried it that way once and it was so messy that I abandonned the idea.
Another way is to create extra tracks in the step sequencer just to hold the triplets, then adjust the swing and timing values for those rows. Again, not exactly quick or elegant.
An alternative is to sequence in the piano roll editor which can handle triplets and anything else you throw at it. Though there are a few things the step sequencer can do that the piano roll can't. Like the step play probability.
As for quavers, semi-quavers etc. you can set the step sequencer to have between one and four steps per 4/4 beat. So for quavers use 2 steps/beat etc. Personally I usually just use 4 steps/beat all the time, partly because adding in extra steps per beat later (or removing them) can cause a little chaos.