For most transformation, we assume that we can compute only the translation coefficients (ai). The only exception are Euclidean transformation around a single axis of rotation that allow to compute a single scaling and a single rotation coefficient. In all other cases, values of computed coefficients would depend on the assumed order of individual transformation, making them no more than a potentially misleading guesses.
Number of parameters: 2
$$ \begin{bmatrix} 1 & 0 & a_1 \\ 0 & 1 & a_2 \\ 0 & 0 & 1 \end{bmatrix} $$
Number of parameters: 4
$$ \begin{bmatrix} b_1 & b_2 & a_1 \\ -b_2 & b_1 & a_2 \\ 0 & 0 & 1 \end{bmatrix} $$
The Euclidean transformation is a special case, where we can compute rotation (θ) and the single scaling (ϕ) coefficients, as follows: $$ \phi = \sqrt{b_1^2 + b_2^2}\\ \theta = tan^{-1}(\frac{b_2}{b_1}) $$
Number of parameters: 6
$$ \begin{bmatrix} b_1 & b_2 & a_1 \\ b_3 & b_4 & a_2 \\ 0 & 0 & 1 \end{bmatrix} $$
Number of parameters: 8
$$ \begin{bmatrix} b_1 & b_2 & a_1 \\ b_3 & b_4 & a_2 \\ b_5 & b_6 & 1 \end{bmatrix} $$
Number of parameters: 3
$$ \begin{bmatrix} 1 & 0 & 0 & a_1 \\ 0 & 1 & 0 & a_2 \\ 0 & 0 & 1 & a_3 \\ 0 & 0 & 0 & 1 \end{bmatrix} $$
Number of parameters: 5
For all Euclidean rotations, we opted to use coefficient b3 to code scaling (ϕ), whereas b2 = sin(θ) and b1 = ϕ cos(θ). The coefficients are computed as follows: $$ \phi = \sqrt{b_1^2 + b_2^2}\\ \theta = tan^{-1}(\frac{b_2}{b_1}) $$
Note that during fitting ϕ is computed from b1 and b2 on the fly. $$ \begin{bmatrix} \phi & 0 & 0 & a_1 \\ 0 & b_1 &-b_2 & a_2 \\ 0 & b_2 & b_1 & a_3 \\ 0 & 0 & 0 & 1 \end{bmatrix} $$
$$ \begin{bmatrix} b_1 & 0 & b_2 & a_1 \\ 0 & \phi & 0 & a_2 \\ -b_2 & 0 & b_1 & a_3 \\ 0 & 0 & 0 & 1 \end{bmatrix} $$
$$ \begin{bmatrix} b_1 &-b_2 & 0 & a_1 \\ b_2 & b_1 & 0 & a_2 \\ 0 & 0 & \phi & a_3 \\ 0 & 0 & 0 & 1 \end{bmatrix} $$
Number of parameters: 12
$$ \begin{bmatrix} b_1 & b_2 & b_3 & a_1 \\ b_4 & b_5 & b_6 & a_2 \\ b_7 & b_8 & b_9 & a_3 \\ 0 & 0 & 0 & 1 \end{bmatrix} $$
Number of parameters: 15
$$ \begin{bmatrix} b_1 & b_2 & b_3 & a_1 \\ b_4 & b_5 & b_6 & a_2 \\ b_7 & b_8 & b_9 & a_3 \\ b_{10} & b_{11} & b_{12} & 1 \end{bmatrix} $$