In terms of cost of living only real difference will be rent - mumbai is costliest, followed by Bangalore while Pune is cheap. Transportation costs will be higher in Bangalore and Pune (unless you have a vehicle); mumbai is cheaper in this regard thanks to metro, busses, trains. But difference in transportation cost will likely be around 5k per month; rental costs can vary by as much as 20k per month between Pune and mumbai.
Other than this all other costs are going to be more or less the same tbh. It depends on you &what kind of life you want to lead. Groceries, zomato/Swiggy, restaurants etc. all cost more or less the same on a monthly basis (unless you use these very frequently)
Lifestyle & vibe in all 3 cities are quite different - mumbai is 24/7 hustle city which is βaliveβ and awake pretty much always; indeed it is the city that never sleeps; while Pune is a lazy sloth that literally takes a nap every afternoon despite waking up late & sleeping early - I kid you not, local shops in Pune are shut in afternoons as people take naps; this kind of behaviour is unheard of in mumbai. Bangalore is somewhere in between with a good mix of both aspects.
Transportation is amazing in mumbai but for BLR and Pune you need to have your own vehicle. Ola/Uber might get expensive. And specifically for Bangalore, Uber/Ola experience is so bad that Iβve had to literally cancel plans with my family multiple times as I could not get any cab. Cab booking experience is so frustrating that Bangalore would make you a recluse reluctant to step out of your home (unless you have a vehicle)
Once you do step out, traffic is bad in all three cities especially at peak office times but with mumbai you get eastern and western express highways which makes for a comfortable travel
Overall all 3 are good cities, you have to figure out which one aligns with your lifestyle