I doubt that a 3KVA UPS will be anything other than a pure sinewave output type. It seems to be a very similar design to that SmartUPS2000 of mine. That uses a class D amplifier to drive conventional 50/60Hz mains transformers in reverse (low primary input, mains secondary output) from a 50 or 60Hz sinewave reference.
It's the type of design that's referred to as a "Line Interactive" UPS where a relay is used to switch from mains input to inverter output on full mains failure with additional relay switching of buck and boost tappings to compensate for sags and surges in mains voltage levels, eliminating the need to use battery power for the more modest out of tolerance departures from nominal supply voltage. If I recall correctly, the mains transformer also powers the battery charger circuit as well.
It might seem rather cumbersome to digitally synthesise 50/60Hz power at 28v rms to feed into a large and heavy mains step up transformer but you have to remember that the transformer also serves as a buck/boost device to reduce demands on the battery as well as act as a charging transformer. At these power demands, portability wasn't considered a high priority way back then.
These days, a modern UPS would do away with such large and heavy mains frequency transformers[1] and simply synthesise a floating fullwave "unsmoothed"[2] 325v peak DC voltage (230v rms case) with a
100% 100/120Hz ripple content connected to the load via a fullwave thyristor commutator circuit to reverse the polarity at half the ripple frequency. [1] The smaller, sub 3KVA line interactive types may include a smaller buck/boost autotransformer to retain the battery saving feature of regulating poorly stabilised mains voltage supplies, probably only weighing a tenth to a fifth of the weight of an Old Skool transformer design like the SmartUPS2000. [2] "Unsmoothed" only in respect of the 100/120Hz 'ripple', smoothed in regard of the HF inverter pulses from the 'class D amplifier'.