they all r in different categories.its not a brilliant thing to compare these cars.
Performnce:Premio is a heavy car so its not having much better acceleration than axio.Axio has a much better power to weight ratio and has superb pick according to engine size.
Belta is very much underpowered when vitz is underpowered.
Parts:Belta will have better availbility spares than two of them.
Cng:belta is cvti so dont expect cng to be good ,dont know about premio but old one was having superb performance on cng,I have a axio running on cng and till now no problem caused and have installed lembda also.Giving 190km on highway with 60kg cylinder and 135+ in city.