What child development theory includes all three,Biological Development,Psychological Development

Are you looking for the third?
That would be the sociological development. The three together are looked as a biopsychosocial model.

For our assignment we done child observations and now we have to find only one theorist and theory that talks about all three developments. Im finding it really hard to find one theory.

Try looking at some of Dr. Benjamin Spock's writings.