It seems all three have some part to play in the advance of autism, plus, doctors nowadays are more aware of the symptoms and will diagnose more cases. Lots of recent studies do show that environmental factors at conception or embryonic stage can increase the risk of autism developing, especially if the genes that code for the brain are involved.
Other research into chemicals - notably BPA - bisPhenol (widely used in plastic containers and food containers/liners) and phthalates (shampoos and soaps) shows that these affect learning,behaviour and memory in zebrafish studies!!
It all gets very complex.
Still further research into sperm and mutations reveals that older males are more at risk of conceiving autistic children, and that males are more likely to be affected - father to son. This is not bound to happen but the chances are increased with age.
I work with autistic young people and have long been reading reports from all kinds of scientific institutions keen to prove a definitive cause. None has yet been able to say with 100% certainty that if a happens to b autism is bound to develop. There are just too many unknowns at the moment.
What is troublesome is the language some scientists use - describing autism as a glitch, a mistake, and other such things, when autistic people want to be known as 'perfectly healthy with a brain which is not neurotypical.'