Changing a model in to a mixed integer linear programing model Trying to learn about integer programming in quarantine and I've come across a problem that stumped me. I searched the net but couldn't see anything similar and would appreciate another set of eyes on how to approach it.

Turn the given model in to a binary mixed integer linear programing
model:
$\operatorname{Max} z=a(x)+2 b(y)$
s.t $\quad x, y \geq 0$
At minimum two thirds of the given constraints apply:
$$2 x+y \leq 16, \quad x+y \leq 9, \quad x+3 y \leq 12$$
$$a(x)=\begin{cases}10+3 x, & \text{if $0 \leq x \leq 4$}, \\ 14+2 x, &\text{if $x \geq 4$},\end{cases} \quad 
b(y)=\begin{cases}8+y, &\text{if $0 \leq y \leq 3$} \\ 2+3y, &\text{if  $y \geq 3$}\end{cases}$$

It hints to consider making use of multiple $x$ and $y$ variables and I know that if I want to try linearizing the problem I should go with $b(y)$ due to $y$ having a coefficient of $3$ in the third function.

There are many different ways to do this, there are different options here. Here is one, which is definitely not the best, but which is quite natural I guess.
I will only show you how to it for $a(x)$, the method is identical for $b(y)$.
Define a binary variable $\delta$ that takes value $1$ if and only if $x\le 4$:
$$
4(1-\delta)\le x \le 4 +M(1-\delta)
$$
$M$ is an upper bound on $x$. And then define the continuous variable $a$ as follows:
$$
a = (10+3x)\delta + (14+2x)(1-\delta)
$$
Now you have non linear terms (in $x\delta$) that you need to linearize, for example like this.

In practice, before using such transformations blindly, be sure to analyze the nature of the function: is it convex or concave, and are you maximizing or minimizing. As mentioned by Rob Pratt, if you are minimizing (maximizing) a convex (concave) function, a much simpler approach is available. Be sure to check his answer.
For the part where at least $2$ constraints out of $3$ should be satisfied, also please refer to @Rob Pratt's answer, as to my knowledge there is no other way to do it.

Just for pleasure, I also like this method:
Redefine $x$ in terms of new variables $x_1 \ge 0$ and $x_2 \ge 0$ as follows:
\begin{align}
x&=x_1+x_2+4(1-\delta)\\
x_1&\le 4 \delta \\
x_2& \le 5 (1-\delta)
\end{align}
So $x_1$ is the range on the first interval $[0,4]$, and $x_2$ on the second one $[4,9]$.
And so you can rewrite $a(x)$ linearly:
$$
a = 3x_1+10\delta +2x_2+22(1-\delta)
$$
