All three cars listed are very different - the Maruti Suzuki Ertiga is a no-nonsense seven-seater MPV; the XUV3XO is a turbo-petrol compact SUV with excellent ride and handling balance; and the Tata Sierra is a mid-size SUV which is likely to offer the most space and feel the most premium of the lot. You don't seem to have a definitive requirement for a seven-seater since two out of the three cars you've listed can only seat five; the only reason to consider an Ertiga is for its fuel efficiency.
If that is something you can overlook, we'd recommend you shortlist the Mahindra XUV 3XO and Sierra. Out of these, even though the Mahindra car is likely to be punchier and more fun to drive, the Sierra's larger size and more cabin space are likely to deliver a nicer drive and cabin experience, making you feel like you've spent your money well. The Sierra's NA petrol isn't likely to be much more efficient than the Mahindra's turbo-petrol, due to the Tata's heavier kerb weight; however, between the two, it is the one we'd recommend.
