In any case, splitting up workouts will be a less effective use of your time, if only due to needing to warm up three times instead of only once.
If you want to do strength training, 20-30 minutes per session will likely not be enough time for long-term development (but probably okay in the short term). For example, to do a set of heavy squats, you will do a general warm-up for 2-5 minutes (stationary bike), then wait for the squat rack to be free, then do a set of 5 reps with the empty bar, a few sets of 1-5 reps with increasing weight as you approach your target weight for the day, then 3 sets of 5 reps each at a heavy weight, requiring 3-7 minutes rest between each set to properly recover for the next set.
Since you're a novice, I'm sure you'll be able to make some initial strength gains in 20-30 minute blocks (since you won't need as many warm-up sets, and rests between sets will be short), but you won't be able to continue that progress for very long. As you gain ability to lift heavier weights, your body will need proper warm-up sets and longer rests between sets, and trying to make that fit into 20-30 minutes just won't work. Maybe your classes will have ended by the time you progress to this stage, though.
If you want to do cario, you can easily make a lot out of 20-30 minutes.
- Warm up for about 5 minutes, and then do hard interval sessions, 3 minutes at about 80% of your fastest speed, and then take 3 minutes at a recovery pace (but still jogging), and repeat.
- Just straight jog for 30 minutes.
- Run 5k.
You can also use 20-30 minutes to do stretching or foam rolling to assist in your muscle recovery and help prevent injuries.