Describe three modifications that a mainstream school can make towards true inclusion for a child?

To be honest, it really depends on the person (autism is tricky like that, so many variables)... but as a high functioning autistic person I can share my own experience. I hope this is helpful. I'm not a teacher, but I am a student.

I think the most important thing is to allow an autistic child a safe space to calm down. A lot of social situations make autistic people very stressed and in class conditions this stress is often amplified. If you provide them with something they enjoy (a good book, their phone or iPad, toys, etc.) they will eventually calm down. This will prevent a lot of meltdowns and disruption in classrooms if you allow them to go to their space when they need it. When I was younger, and even sometimes now, a safe space is important for autistic people not to freak out.

Another modification could be extension tasks... This obviously wouldn't apply to everyone but it can be helpful to autistic people who are very smart and get bored easily with the task on hand. A more difficult task could allow them to stretch themselves and enjoy class a little more. Also if they are busy working on something they find challenging, they will be less disruptive to the class (autistic people are very focused and passionate when they are interested in what they're doing.)

Lastly, maybe just talking to them. Again, this one isn't universal. But I have found talking to adults at difficult times in my life (especially during struggles with school) was awesome. I think the most important thing is that they know you are trying to understand their disability and will listen and help them with their problems. All autistic people are different, and the most effective thing you can do is build their trust and listen to them.

Hope this helped, sorry for the essay ;)