Let's simplify the given expressions step by step.

### Expression 1: \( 3 - x^2 \)

This expression is already in its simplest form. It represents a quadratic expression where the coefficient of \( x^2 \) is \(-1\), and the constant term is 3.

### Expression 2: \( 3 - (x + 1)^2 \)

First, let's expand the squared term:

\[ (x + 1)^2 = x^2 + 2x + 1 \]

Now, substitute this back into the expression:

\[ 3 - (x + 1)^2 = 3 - (x^2 + 2x + 1) \]

Distribute the negative sign:

\[ 3 - x^2 - 2x - 1 \]

Combine like terms:

\[ 3 - 1 - x^2 - 2x = 2 - x^2 - 2x \]

So, the simplified form of the second expression is:

\[ 2 - x^2 - 2x \]

### Summary

- The first expression \( 3 - x^2 \) is already simplified.
- The second expression \( 3 - (x + 1)^2 \) simplifies to \( 2 - x^2 - 2x \).