Let's simplify this, so say if we were to sign those three players and they would stay here until they were 33. Bale would give us 4 seasons and would cost say £60m, so that'd cost us £15m a season. Griezmann would give 6 seasons and would cost £90m, so that's again £15m a season. Dybala will give 9 seasons and would cost around £130m, so that's about £14.5m a season. So all pretty similar if we break it down like that.
However, a big difference is that with Dybala in say 3/4 seasons, he'd be 27/28 and likely peaking, so if we do decide to sell then, we should be able to get a very good fee back (similar to how Madrid bought a 24 year old Bale and may sell him on 5 years later for a similar fee to the then world record fee they paid). You wouldn't be able get the same fee back with Griezmann and most definitely not with Bale.
That was all taking this from a purely financial perspective, but if we were look at play style, then Bale is someone who is heavily reliant on pace (something that is expected to decrease once players approach 30). However, Dybala and Griezmann are both players that are less reliant on pace and more on technical ability. Hence, one should expect both of them to be more likely play at a higher level even once they are in their mid 30's.