why on earth would you say ASD cannot be diagnosed until 3?! what utter bollocks (to put it mildly)
dd1 was dx'd at 2.7, having been in the system since she was 20 months old. she should have been dx'd a LOT earlier - all the info was there, but the nhs/early years like to delay, for little reasona lot of the time.
you are right that an early dx does not lead to early intervention - dd1 has never had OT or SALT offered (many assessments, no actual therapy on the SALT side, no assessment for OT). she is now nearly 7, so has been dx'd for well over 4 years, and is fully statemented, yet still no actual intervention (well, she has had plenty, all privately sourced and paid for, but ykwim)
BUT this is not a reason to not dx early - it is a reason to sort out early intervention.
there are some impressive stats for early intervention (am talking specifically wrt ASD issues here, as that is my area of interest!), especially from America - children who receive proper early intervention (not just parental education and coffee mornings) go on to achieve higher standards than ever predicted for them.
I do notunderstand why it shoudl be an issue that anyone would look out for issues as early as possible, and seek to rectify those issues as early as possible.
mind you, I was treated like an utter freak for daring to point out to my hv that dd1 was not progressing typically when she was 10 months old - they kept insisting she was lovely (she was, and is), and that I was lucky to have such a beautiful, well behaved baby - err, she was totally uninterested in anything at all - not people, toys, pictures, nothing. she was late meeting all milestones, and made little eye contact. she turned away from touch, and could not even chew food at 14 months+ - now THAt is stressful: repeatedly trying to tell professionals that you think there is something wrong with your child, and having them turn you away as it is "too early to tell" 
a good, cohesive programme to pick up on any early difficulties, and point to appropriate interventions before school starts would be fantastic, imo.