$\omega$ is a $p$-form over $\mathbb{R}^3$ in each example.
$p=0$:
The following formula holds by definition and can't be simplified further:
\begin{align}
\omega =\ &f\\
\Rightarrow d\omega =\ &\sum_i \frac{\partial f}{\partial x_i} dx_i\\\\
\omega =\ &xy - xz + z\\
\Rightarrow d\omega =\ &\frac{\partial}{\partial x}\left(xy - xz + z\right)dx + \frac{\partial}{\partial y}\left(xy - xz + z\right)dy + \frac{\partial}{\partial z}\left(xy - xz + z\right)dz\\
=\ &\left( y - z \right)dx + xdy + (1-x)dz
\end{align}
$p=1$:
Remember that $dx_i \wedge dx_j = - dx_j \wedge dx_i$ and consequently $dx_i \wedge dx_i = 0$. The following formula holds by definition:
\begin{align}
\omega =\ &\sum_{i} f_i dx_i\\
\Rightarrow d\omega =\ &\sum_i \sum_j \frac{\partial f_i}{\partial x_j} dx_j \wedge dx_i\\\\
\omega =\ &+ (x-y)dx\\
&+ zdy\\
&+ (1-z)dz\\\\
\Rightarrow d\omega =\ &+\frac{\partial}{\partial x}(x-y)dx\wedge dx &&+ \frac{\partial}{\partial y}(x-y)dy\wedge dx &&+ \frac{\partial}{\partial z}(x-y)dz\wedge dx\\
&+ \frac{\partial}{\partial x}(z)dx\wedge dy &&+ \frac{\partial}{\partial y}(z)dy\wedge dy &&+ \frac{\partial}{\partial z}(z)dz\wedge dy\\
&+ \frac{\partial}{\partial x}(1-z)dx\wedge dz &&+ \frac{\partial}{\partial y}(1-z)dy\wedge dz &&+ \frac{\partial}{\partial z}(1-z)dz\wedge dz\\\\
=\ &+0 &&+ \frac{\partial}{\partial y}(x-y)dy\wedge dx &&+ \frac{\partial}{\partial z}(x-y)dz\wedge dx\\
&+ \frac{\partial}{\partial x}(z)dx\wedge dy &&+0 &&+ \frac{\partial}{\partial z}(z)dz\wedge dy\\
&+ \frac{\partial}{\partial x}(1-z)dx\wedge dz &&+ \frac{\partial}{\partial y}(1-z)dy\wedge dz &&+0\\\\
=\ &+ \frac{\partial}{\partial x}(z)dx\wedge dy &&+ \frac{\partial}{\partial y}(x-y)dy\wedge dx\\
&+ \frac{\partial}{\partial x}(1-z)dx\wedge dz &&+ \frac{\partial}{\partial z}(x-y)dz\wedge dx\\
&+ \frac{\partial}{\partial y}(1-z)dy\wedge dz &&+ \frac{\partial}{\partial z}(z)dz\wedge dy\\\\
=\ &+ \frac{\partial}{\partial x}(z)dx\wedge dy &&- \frac{\partial}{\partial y}(x-y)dx\wedge dy\\
&+ \frac{\partial}{\partial x}(1-z)dx\wedge dz &&- \frac{\partial}{\partial z}(x-y)dx\wedge dz\\
&+ \frac{\partial}{\partial y}(1-z)dy\wedge dz &&- \frac{\partial}{\partial z}(z)dy\wedge dz
\end{align}
\begin{align}
=\ &+ \left( \frac{\partial}{\partial x}(z) - \frac{\partial}{\partial y}(x-y) \right) dx\wedge dy\\
&+ \left( \frac{\partial}{\partial x}(1-z) - \frac{\partial}{\partial z}(x-y) \right) dx\wedge dz\\
&+ \left( \frac{\partial}{\partial y}(1-z) - \frac{\partial}{\partial z}(z) \right) dy\wedge dz\\\\
=\ &- dx\wedge dy\\
&+ 0\\
&+ dy\wedge dz\\\\
=\ &dy\wedge dz - dx\wedge dy
\end{align}
This formula can be simplified:
\begin{align}
\omega =\ &\sum_i f_i dx_i\\
\Rightarrow d\omega =\ &\sum_i \sum_j \frac{\partial f_i}{\partial x_j} dx_j \wedge dx_i\\
=\ &\sum_{i < j} \left( \frac{\partial f_j}{\partial x_i} - \frac{\partial f_i}{\partial x_j} \right) dx_i \wedge dx_j
\end{align}
Notice how applying the simplified formula skips nearly all the steps in the computation above.
$p=2$:
\begin{align}
\omega =\ &\sum_{i < j} f_{ij} dx_i\wedge dx_j\\
\Rightarrow d\omega =\ &\sum_{i < j} \sum_k \frac{\partial f_{ij}}{\partial x_k} dx_k \wedge dx_i \wedge dx_j\\
=\ &\sum_{i<j<k} \left( \frac{\partial f_{jk}}{\partial x_i} - \frac{\partial f_{ik}}{\partial x_j} + \frac{\partial f_{ij}}{\partial x_k} \right) dx_i \wedge dx_j \wedge dx_k
\end{align}
$p=3$:
\begin{align}
\omega =\ &\sum_{i < j < k} f_{ijk} dx_i \wedge dx_j \wedge dx_k\\
\Rightarrow d\omega =\ &\sum_{i < j < k} \sum_l \frac{\partial f_{ijk}}{\partial x_l} dx_l \wedge dx_i \wedge dx_j \wedge dx_k\\
=\ &\sum_{i<j<k<l} \left( \frac{\partial f_{jkl}}{\partial x_i} - \frac{\partial f_{ikl}}{\partial x_j} + \frac{\partial f_{ijl}}{\partial x_k} - \frac{\partial f_{ijk}}{\partial x_l} \right) dx_i \wedge dx_j \wedge dx_k \wedge x_l
\end{align}
$p \in \mathbb{N}_0$:
\begin{align}
\omega =\ &\sum_{i_1 < \dots < i_p} f_{i_1\dots i_p} dx_{i_1} \wedge \dots \wedge dx_{i_p}\\
\Rightarrow d\omega =\ &\sum_{i_1 < \dots < i_p} \sum_j \frac{\partial f_{i_1\dots i_p}}{\partial x_j} dx_j \wedge dx_{i_1} \wedge \dots \wedge dx_{i_p}\\
=\ &\sum_{i_1 < \dots < i_{p+1}} \left( \sum_{j=1}^{p+1} (-1)^{j-1} \frac{\partial f_{i_1\dots i_{j-1}i_{j+1}\dots i_{p+1}}}{\partial x_{i_j}} \right) dx_{i_1} \wedge \dots \wedge dx_{i_{p+1}}
\end{align}
Best Answer
If I understood your question correctly, we can discuss first how to pullback a single wedge of $dx$s, then generalize by linearity. Let $\omega = dx_I = dx_{i_1}\wedge \dots \wedge dx_{i_k}$ and $\phi(u_1,\dots,u_m):\mathbb{R}^m\rightarrow\mathbb{R}^n$ the smooth map, then
$$\phi^*\omega =\bigwedge_{1\leq j\leq k} \sum_{q_j=1}^m \frac{\partial\phi_{i_j}}{\partial u_{q_j}}du_{q_j}=\sum_{1\leq q_1,\dots,q_k\leq m} \prod_{j=1}^k \frac{\partial\phi_{i_j}}{\partial u_{q_j}}du_{q_1}\wedge \dots du_{q_k}$$
Now for every $Q=(q_1,\dots,q_k)$ if two $q$s are the same we get $0$. Otherwize, there exists a unique increasing multi-index $J$ and a permutation $\sigma$ such that $\sigma(J)=Q$. If we sum according to $J$s and $\sigma$s, we get
$$\sum_{J}\sum_{\sigma \in S_k} \prod_{j=1}^k \frac{\partial\phi_{i_j}}{\partial u_{q_{\sigma(j)}}}du_{q_{\sigma(1)}}\wedge \dots du_{q_{\sigma(k)}} =\sum_{J}\sum_{\sigma \in S_k} \prod_{j=1}^k \frac{\partial\phi_{i_j}}{\partial u_{q_{\sigma(j)}}}\text{sgn}\sigma \ du_J$$ Note that by the permutation property of determinant we get the determinant created by the $i_1,\dots,i_k$ rows and $j_1,\dots,j_k$ columns of $D\phi$. We can write this as
$$\sum_J \det \frac{\partial \phi_I}{\partial u_J} du_J$$ Remember $J$ runs over all increasing multi-indexes of length $k$ from $1,\dots,m$.