I think what we are saying is that yes you can diagnose ASD under 3 and many of us got dx under 3, but it would probably not bring anything extra in terms of help. Portage+SALT or autism outreach+SALT is basically all that is on offer for under 4's. You may get some 1: support at a mainstream nursery assuming you would want a nursery place.
Many services only want to work (and only resourced to work) with nurseries and schools to give them 'advice' they do not want to work with the family or child directly.
My child was dx at 2.5 with massive regression and moderate-severe autism. He lost 18 months worth of skills. They sent an autism teacher to his nursery once a term who told the nursery to use symbols, buy a white pop up tent, take anything bright off the walls and draw lines on the floor to teach him not to escape out the door. That was it. No programmes, no strategies, no demonstration of techniques. She did not even interact with my child. Ditto SALT came a few times and said hold things by your face so he looks at you, use one word instructions, roll a ball back and forth. That was it. No targets, no programmes, no proper assessment. That was his provision for 18 months. needless to say he made zero progress.
Portage is worth doing (we were refused portage as you could not have both portage and autism outreach - so here people delay getting a autism dx so they do not lose portage!).
In terms of autism provision you will either need to teach yourself or go private, at least until you can get into a specialist nursery or programme.
Look at Hanen More than Words book (Wilmslow best price) which is good easy read for where to start with strategies. Then read up on ABA (applied behaviour analysis), Floortime, RDI, Early Denver etc. In USA ABA is the standard approach used and modern ABA is becoming more mixed with play based approaches eg verbal behaviour, Early Denver, PRT. You will find out more reading websites of US autism centres / Paediatricians than UK ones.
Find out about specialist schools, nurseries, ASD or speech units near you (State or private) and apply now for a statement of SEN. We applied at 2.7 and got a final statement after appeal at 3.3 which we then had to appeal as it was for mainstream and Ed Psych had advised specialist - we finally won ASD specific provision at Tribunal at 3.11.
If you can afford private therapy e.g. SALT or applied behaviour analysis then do that. We started ABA after 6 months of waiting for someone to do something. We were told by the SEN officer at the council 'its just autism, we would expect him not to speak or socialise, thats not enough for specialist support he would need to have a feeding tube as well'! They just tried to drag the process out until they could dump all responsibility on a mainstream school or nursery.
Just because health professionals can diagnose at 2 or 3 does not mean local councils have changed their provision to accommodate younger children. Most councils still have services geared at over 5's and little to offer younger children.
And NHS SALT has been useless. Most NHS SALTS have one day training on autism, they know how to teach speech sounds but they don't know how to motivate a child to speak i.e. the autism bit of communication. So often its a dead loss.
You could look at something like PECS - I went on a PECS course early on and found that useful (Look up Pyramid PECS for workshops).
You should push for a diagnosis but it will not in itself bring any additional help. The best way to access specialist provision is to apply for a Statement and prepare yourself to appeal at each stage you are turned down - which at 2 will be every stage probably.
You can also self refer to an educational psychologist for advice on appropriate provision so get on this list as well.
For Statements of SEN look at IPSEA. Just send off the standard letter. They will turn down the request for assessment as premature - so you appeal - then the council has to gather evidence to defend the appeal and show the tribunal your child's needs are being met - so at that point they will need to send in autism teachers / early years specialist teachers and Ed Psychs + get advice from a paediatrician / SALT.
Find out if you are eligible for legal aid (legal aid website) if you have to go to appeal - if so legal aid will pay for private reports in an appeal
It won't make you popular applying now - you will be told you are jumping the gun and should just wait until all these wonderful services come in. Trust me, don't listen and don't wait. The way it was described to me was that this amazing swat team was going to come in and provide all this support. Then all we got was advice you could read yourself from a £5.99 paperback on autism and a tent. When we expressed surprise that 'this was it' we were vilified, bullied and labelled obstructive and dishonest in the hope a Tribunal would dismiss our evidence.
My son made more progress in his first week of ABA than he did in a year of the State provision. Early intervention does make a big difference and whatever you can learn to do yourself or pay someone specialist to teach you will be worth 100 times what a diagnosis will bring.