There are different levels of autism and if the child who has autism is not severely autistic then the child can usually be potty trained.
But if the child has severe autism then the child may not be able to learn to use the potty and may need to wear diapers for the rest of their life.
Children with autism and other disabilities can sometimes be potty trained as long as the child's disability is not very severe.
If they do have some disability though including autism the child may take longer to potty train and may not be ready to potty train until 4 years old or even as late as 8 years old in some kids.
If the child isn't ready to potty train even at 4 and they have a disability then you should not push it and continue to diaper them.
But if the child has no disability or is not incontinent then they should be potty trained by 4 at the latest.