When you put a child with speech delay age 3 in a class of autistic children can they pick up their behaviors?

Possibly, however, Speech = articulation, the motor aspect of language. A child with receptive & expressive language delay" has impaired ability in comprehending language & in verbalizing ideas. Language is best learned from birth-5. Autism pre-k has 6 hrs. Of intensive instruction. Developmental delay pre-k has 2 1/2 hrs. Of instruction for children with various needs. Try the higher level of service first.
Yes. A three year old child might mimic anything they see and are repeatedly exposed to. This applies to both good and bad behaviors, and to speech. That's why it's important that good behaviors (and language) are modelled for children from a very young age.
Yes. If the child with speech delay is not autistic, they should not be in a class with autistic children, since they may pick up the behaviors of others. Instead they should be in a preschool class with other children their age, hopefully a mix of children with normal and delayed speech patterns, which have been shown to be beneficial approach.
Yes. It is important to properly place a child with speech delay as they may mimic the behavior of the other children in their class.