Work out the coefficients first: 1/2, 1/2(-1/2)/2, 1/2(-1/2)(-3/2)/6 = 1/2, -1/8, 1/16.
The x terms are -2x+3x^2, (-2x+3x^2)^2, (-2x+3x^2)^3 = -2x+3x^2, 4x^2-12x^3+9x^4, -8x^3+36x^4-54x^5+27x^6
So the first four terms are:
1-x+3x^2/2-x^2/2+3x^3/2-9x^4/8-x^3/2+9x^4/4-27x^5/8+27x^6/16 =
1-x+x^2-x^3+9x^4/8-27x^5/8+27x^6/16 or 1-x+x^2-x^3 if you mean the first four terms of the polynomial.