Trouble understanding proof of the inequality - $(\frac{1}{a}+1)(\frac{1}{b}+1)(\frac{1}{c}+1) \ge 64 $, for $a,b,c > 0$ and $a+b+c = 1$ I was looking into this problem in a book discussing inequalities, However I found the proof quite hard to understand.The problem is as follows:

Let $a,b,c$ be positive numbers with $a+b+c=1$, prove that
$$\left(\frac{1}{a}+1\right)\left(\frac{1}{b}+1\right)\left(\frac{1}{c}+1\right) \ge 64$$

and the proof provided was the following:
Note that $$ abc \le (\frac{a+b+c}{3})^3 = \frac{1}{27} \tag{1}$$
by AM-GM inequality. Then
$$(\frac{1}{a}+1)(\frac{1}{b}+1)(\frac{1}{c}+1)=1+\frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{ab}+\frac{1}{bc}+\frac{1}{ca}+\frac{1}{abc} \tag{2}$$
$$\ge 1+\frac{3}{\sqrt[3]{abc}}+\frac{3}{\sqrt[3]{(abc)^2}} +\frac
{1}{abc} \tag{3}$$
$$=(1+\frac{1}{\sqrt[3]{abc}})^3 \ge 4^3 \tag{4}$$
Steps 1 and 2 are easy for me to understand, but if someone could help me with steps 3 and 4 ,I would be very thankful.

For Step $(3)$,
Using AM-GM inequality
$$\frac{1}{a}+\frac{1}{b}+\frac{1}{c} \geq 3 \sqrt[3]{\frac{1}{a} \cdot \frac{1}{b} \cdot \frac{1}{c}}=\sqrt[3]{\frac{1}{abc}}\tag{3.1}$$
$$\frac{1}{ab}+\frac{1}{bc}+\frac{1}{ca} \geq 3 \sqrt[3]{\frac{1}{ab} \cdot \frac{1}{bc} \cdot \frac{1}{ca}}=\sqrt[3]{(abc)^2}\tag{3.2}$$
For Step $(4)$, 
Setting $a=1$ and $b=\frac{1}{\sqrt[3]{abc}}$ and using $a^3+3a^2b+3ab^2+b^3=(a+b)^3$ we get, 
$$1+\frac{3}{\sqrt[3]{abc}}+\frac{3}{\sqrt[3]{(abc)^2}}+\frac{1}{abc}=\left(1+\frac{1}{\sqrt[3]{abc}} \right)^3\tag{4.1}$$
Using AM-GM inequality
$$\frac{a+b+c}{3}=\frac{1}{3} \geq \sqrt[3]{abc} \Rightarrow \frac{1}{\sqrt[3]{abc}} \geq 3 \tag{4.2}$$ as $a,b,c>0$ and the result follows.
