$\Phi_1=1$ or $\Phi_1=2$ for the dynamic $\text{Table-Insert}$ , where $\Phi_i$ is the potential function after $i$ th operation, as per CLRS
You have caught an instance of the infamous off-by-one error in that popular textbook whose name we shall not mention again.
To repeat, it is correct that &quot;the cost $c_1=1$, $\Phi_0=0$&quot;, &quot;$num_1=size_1=1$ $\implies$ $\Phi_1 = 2\cdot1-1 =1$&quot; and &quot; $\hat{c_1}=$ $c_1+\Phi_1-\Phi_0$ $=2$&quot;. It is incorrect to state that $\widehat c_i=3$ for all $i$.
The first $\text{T}\scriptsize{\text{ABLE}}\small\text{-I}\scriptsize\text{NSERT}$ operation is indeed very special. It is not considered as an expansion, an event that is defined as &quot;in which lines 5–9 are executed&quot;. However, it does not maintain $size_i = size_{i-1}$, either. So neither computation for $\widehat{c_i}$ in the textbook is fit for $\widehat{c_1}$.
That error is confusing given that we tend to trust fully an exemplary textbook where so much attention have been paid to detail and correctness. 
On the other hand, that error is not very significant since, as you noted, it still holds that &quot;the amortized cost is at most 3&quot;. 
By the way, if you take a closer look at the figure at the end of question, it shows the potential at the end of first $\text{T}\scriptsize{\text{ABLE}}\small\text{-I}\scriptsize\text{NSERT}$ operation, $\Phi_1=1$, the correct value.